Protoss V.S Protoss

Overview
As with any same species encounters, Protoss vs. Protoss battles are more a race for resources and unit production than anything else. The capabilities at hand are the same for both sides so the player that knows his/her force the best, both their strengths and weaknesses, will usually be the victor. Zealot Rush
In most Protoss on Protoss encounters, the first few minutes of the game will usually consist of Zealot Rushing (at least on land maps). These encounters will typically boil down to numbers or micromanagement. If you find yourself outnumbered when rushing into another player's base, your best option is to go straight for your opponent's Probes and slow down the resource production as much as possible. Make sure you command your Zealots to attack the Probes directly and not with an Attack-Move command to ensure the fastest and most efficient kill rate. Try and avoid direct Zealot engagements in this type of situation since not only are you outnumbered, but you will be unable to reinforce your troops in time, while your enemy will have their Zealots producing in close proximity. If you find yourself outnumbered in your own base, try and initiate Zealot on Zealot encounters using Hit and Run tactics until you've produced enough Zealots at your own Gateways to counter the invading force. Try to avoid sending your Probes to attack the Zealots unless it's a last resort or if the enemy Zealots are heavily damaged since doing so would be like sending lambs to the slaughter. If you outnumber the enemy when invading, kill the enemy Zealots as soon as possible then go straight for the mining operation. Reinforce your troops as quickly as possible to make sure the pressure on your enemy doesn't let up and in most situations, you'll be able to finish them off quickly. Watch for escaping Probes, but don't send all of your Zealots chasing after them. It's better to finish off the enemy Nexus first to ensure they can't return to it later on, and even if they have the resources to build another one, it will take some time to Warp it in from Aiur, time you can use to find and hunt down the wandering Probe(s) after you've finished off their original base. If you outnumber the enemy on defense in your own base, engage the invasion force as quickly as possible and do all you can to prevent them from reaching your mining Probes. Zealots and Dragoons
If you make it past the early game Zealot rushing, you'll want to begin supporting your Zealots with Dragoons. They're useful for getting free hits in during Zealot on Zealot encounters and in numbers with the range upgrade, they can heavily damage Zealot forces before they can even reach the Dragoons. Also if your Dragoons survive into the late game, they serve as excellent anti-air units and are particularly useful for fending off the Reaver Drop. In the latter game, you may want to concentrate production more on Dragoons than Zealots, but the combination of the two is undeniable throughout the entire game, so don't neglect to make a few Zealots even in the late game. Detection
With the Dark Templar rejoining their Protoss brethren on the battlefronts, detection is an important issue in the early-mid game. The need for Photon Cannons is essential for base defense and you'll need them in numbers. If possible, keep a small cluster of Photon Cannons (since a single Cannon is almost useless against even a single Dark Templar) close to your mining operation and watch and listen for signs of Dark Templar attack. Other than the Dark Templar, the only units with cloak abilities are those that are within an Arbiter's field. You can combat that method of cloaking either by chasing away or destroying the enemy Arbiter or have an Observer/Photon Cannons nearby, but with the cost of Arbiters as well as their production position in the tech tree, you probably won't encounter them in the majority of your battles. ScoutCraft
Most of the Protoss ground units have difficulty handling aerial units (except maybe for the Dragoon and Archon) and the Scout can be especially useful in taking advantage of that problem. Not only can they attack both ground and air targets, but their speed and manuverability make them the perfect unit for scouting around the map and quickly shutting down enemy expansion attempts (which is extremely important considering the speed at which a single Probe can set up a stalwart defense and mining operation). Their speed also comes in handy for quick retreats to Shield Batteries and moving back to the forefront of battle. Once gathered in numbers, Scouts can be deadly against any force, and the more upgraded they are, the more destruction they can cause. Groups of 18 or more Scouts are extremely difficult to eliminate, even with the mighty Carriers, but be sure to keep them together for maximum efficiency. Reavers
The Reaver is a powerful weapon against any enemy Protoss force. Use the Reaver Drop to quickly destroy mining Probes at enemy bases. They're also very effective for quickly destroying enemy Shield Batteries and Photon Cannons. Tightly packed enemy ground units are also prime targets for Reavers. However, with their slow movement, be very wary of enemy Psionic Storms as well as Scouts and Carriers. Arbiters
If you do get the opportunity to produce Arbiters, you can gain certain advantages in battle. If you can clear out an enemy's detectors, by destroying their Observers and/or Photon Cannons, units beneath the Arbiter's cloak field will be able to quickly destroy an enemy force or base. However, an even better use of the Arbiter lies in the ability to bypass enemy defenses with an Offensive Recall. This will allow you to quickly transport troops right into the middle of an enemy base, or across the map in an instant and without having to risk losing units inside Shuttles making a run past a Photon Cannon defense. The Arbiter's Stasis Field ability is also very useful against groups of Carriers, giving you enough time to set up the proper countermeasures. Psionic Storm
The High Templar's Psionic Storm ability is probably the most effective tool to use later in the game against other Protoss. It causes massive damage to every Protoss unit, especially if they're packed together. Casting Psionic Storm over a busy mining operation can reduce the enemy colony's output in half (if not more) by killing all the Probes beneath it. It's also one of the better counters against a Reaver Drop (especially if you can manage to hit both the Shuttle escort and its Reavers in one cast. Also against large fleets of Carriers, Psionic Storm can help save your base by heavily damaging all Carriers within the area (which is fairly easy since the Carriers tend to "stack" on top of each other) as well as the attacking Interceptors. Psionic Storm is also key when defending your base, since Psionic Storm has no effect on buildings. Make generous use of it should you get invaded by Zealots, Dragoons, Carriers, etc. Hallucination
The High Templar's Hallucination ability is also an effective tool to use against other Protoss. Use it with Shuttles to give them a better chance of surviving when running past a blockade of Photon Cannons. Hallucination also works well with Carriers, but usually with about a 1:1 ratio between Hallucinated and real Carriers, since 8 Hallucinated Carriers escorting only 1 real Carrier can look funny when only the 1 real Carrier begins deploying Interceptors. You can also use Hallucination to "enlarge" an Archon force giving your real Archons more time to dish out the damage.

Protoss V.S Teren

Overview
The Terrans are primarily a species built for a strong defense while the Protoss are built for superior power. If you hope to defeat an enemy with an edge defensively, you'll need to overpower them offensively. If you fail to keep the pressure on a Terran opponent, you'll often find yourself facing well fortified enemy expansions and bases that are difficult to break through and if they bring that defensive front to your front door, preventing you from expanding or scouting, all the superior power at your disposal will not be able to overcome a disadvantage in resource production. Zealot Rush vs. Marine Rush
As is typical in most games, the first few minutes of the game will most likely involve rushes. The Protoss Zealots can easily overpower Terran Marines (they can take two Marines for every one Zealot) but with the cost and build time difference between the two units, Zealots have to be wary of being vastly outnumbered. When engaging, try and avoid group selecting all of your Zealots and commanding to attack a single Marine since that will most likely result in your Zealots wandering around, trying to circle around each other to reach the target while the other enemy Marines get their free hits in from a distance. Instead, make ample use of the Attack-Move command to have your Zealots quickly attack the nearest enemy Marine. If you choose to invade with a Zealot Rush, keep an eye out for Bunkers filled with Marines. Once in Bunkers, it is the Marines that hold the advantage with lesser numbers. If you wish to directly confront the Bunkers, make sure you bring enough Zealots (about 2 Zealots for every Marine within a Bunker) to quickly take it down and then kill the Marines within. If your Terran foe happens to build Bunkers away from their actual mining operation, use the Bunker's static position against it by walking past, ignoring the damage taken along the way, and directly attack the mining SCVs. With this type of movement, make sure you are only using the Move Command and not the Attack-Move as you head into the Terran base or else you may find your Zealots being torn apart by Marine fire while they attempt to pound on the Bunkers. In the case that your opponent has built their Bunkers close to their Command Center and SCVs, the best option is to pull back and wait until you have enough Zealots or more advanced units to overwhelm the enemy. Sneak Attack
One major Terran weakness is the lack of mobile dectection early in the game. If you can get Dark Templar early enough, you should be able to easily walk into a Terran base and quickly raze the buildings and destroy the units. The are only two things to be wary of when you use an early Dark Templar attack, and they are the ComSat Station Scanner Sweep (which will reveal the Dark Templar within the scanned area for a short time) and the Missile Turret (which provide static dectection, much like the Photon Cannons except they lack the ability to hit ground targets). The Scanner Sweep has a limited detection range and can only be used when the ComSat Station has built up enough energy. If this is all the detection the Terran has when you invade with your Dark Templar, try and force the use of the Scanner Sweep by making a quick single hit and run with a Dark Templar. Hopefully this will cause the Terran to use the Scanner Sweep over their base, but with your Dark Templar well out of the way, revealing nothing. Try this a few more times until you're sure the energy reserves in the ComSat Station have been depleted then walk in and quickly destroy the ComSat Station and then proceed to eliminate your foe. With Missile Turrets, you may need to take a little more caution, particularly if there are more than one around. Like the Zealots against Bunkers, you can attempt to quickly make it past the detection abilities of the Missile Turrets, however, if Bunkers filled with Marines are nearby you may want to retreat and wait for a later opportunity to use your Dark Templar. If there are only one or two Missile Turrets guarding an area, and if you have enough Dark Tempar, quickly destroy the detectors and once again you will be protected in your cloaking abilities as you proceed to raze the enemy. In the chance that the Terran has both Missile Turrets and ComSat defending their base, you probably will not want to just walk your Dark Templar into the base. Instead, look for any gaps in the anti-air defense and proceed with a drop from behind with a Shuttle and Dark Templar to put a stop to the enemy resource production. Handling Siege Tanks
The most potent threat from the Terrans comes in the form of the Terran Siege Tank. These awesome weapons have a staggering attack range and if there are enough of them around, they can quell almost any Protoss ground force. Your best bet is probably with Zealots that have been upgraded with Leg Enhancements (speed upgrade). Since the Siege Tanks cannot fire right next to themselves when in Siege Mode, the Zealot can attack with impunity if they can make it up to the Tanks. If there are several Tanks spread apart, split your Zealot forces into smaller groups and send each group to a different Siege Tank. Not only will this help speed up the process of destroying the Tanks, but also with multiple threats, the Tanks can be "tricked" into firing at the opposing Zealots, causing massive damage against each other (due to their explosive damage type and Large size). However, those tactics will generally only work against Siege Tanks out in the field lightly defended by Marines or Firebats, etc. If your opponent has built up a substantial blockade using Tanks to support Bunkers and Missile Turrets, then the situation becomes much more complex. Your best bet will be to overwhelm the blockade with 4+ Carriers (more if there are Goliaths or air unit support nearby). Target any Missile Turrets first and chase away any nearby Science Vessels, and you can give any Dark Templar you have a chance to destroy the ground base blockade. Avoid using High Templar and Psionic Storm against this kind of fortified defense, since not only will the Siege Tank be able to fire at the High Templar before it can use Psionic Storm, the Storm will also be useless against the Bunkers and Missile Turrets. In the latter game, against blockades, look for holes in the base defense and use an Offensive Recall to quickly overwhelm. This will give your ground units the advantage of being able to reach the Siege Tanks without suffering the damage of a normal ground based approach and it can also give you quick access to whatever the blockade is protecting, whether it be a main base or an expansion. Science Vessels
The Terran Science Vessels are probably the greatest threat for Protoss players. Not only do they serve as mobile detectors, but they also have three key special abilities that can cause massive disarray: EMP Shockwave, Irradiate and Defensive Matrix. EMP Shockwave in particular can be deadly for the Protoss since not only is the energy drained from key units like the High Templar, Arbiter and Dark Archon, it also instantly depletes the Plasma Shields from every Protoss unit and building within its range of effect. As a Protoss player, you'll have to keep a sharp eye for them and hunt them down with Scouts, Corsairs or any other means possible when you see them. Battlecruisers
The Terran Battlecruiser is probably the closest thing to an "ultimate" unit as you can find in StarCraft, and as such you should always keep your eye out for amassing Battlecruiser fleets. There are several ways you can handle them and they do require a lot of micromanagement as well as your complete attention when facing them. First of all, they are slow moving and as such, are good targets for Psionic Storms. While Psionic Storm will not destroy them in a single use, it will help reduce their massive amount of HP. Once Psionic Storm is cast, quickly follow up with anti-air units (Carriers will probably be your best bet) and keep the pressure hard on them. If you give the Battlecruisers the opportunity to return to a base or expansion to be repaired, you are essentially wasting your time against them. You may also want to use Hallucinated Carriers or Arbiters to help throw the Battlecruisers off balance while keeping the heat from your real units and when attacking be sure to concentrate all firepower on one Battlecruiser at a time. This will help you reduce their numbers faster and by killing them quickly, you will find yourself with less to worry about later in the game. However, if you know your Terran opponent is going to build mass Battlecruisers, your best bet is to get as many Dark Archons as possible and research Mind Control. By using Mind Control, you can gain the awesome power of the Battlecruiser for your own use and if you have enough Dark Archons at hand when they attack, you can capture an entire fleet within the span of a few heartbeats.

Protoss V.S Zerg

Overview
The Protoss against Zerg encounters are a literal example of quality over quantity. The Zerg, with their cheaper units, 2 for 1 deals and 3 simultaneous unit production at each Hatchery, can produce massive numbers of units. The Protoss cannot hope to match them unit for unit, however, their superior technology gives them an edge in terms of raw power per unit. The key to defeating the Zerg with Protoss lies in meticulous unit micromanagement, quick expansions (you'll need the resources to produce enough of everything) and making sure every unit lives as long as possible (with the use of Shield Batteries). Zealot Rush vs. Zergling Rush
The Zerg have the fastest rush in the game and depending on the map being played, you may find yourself being overwhelmed before your first Zealot is produced. By the time a rush like that hits, you should at least have one Pylon and a Gateway up with a Zealot beginning production, and in that case gather up all your Probes and lead the Zerglings on a chase around and around until your Zealot is produced. At that point, use the Zealot to attack and use your Probes to help it finish off the offending Zerglings. Once the Zerglings are dead, you'll want to recommence with your resource production as soon as possible. Note that a rush like this, this early in the game will severely hamper a Zerg player's resource production and the key to winning lies in making sure that you can out-resource and out-produce your opponent. At most they will be able to send another wave or two of Zerglings but by that time you should have a few Zealots out and able to defend without too many problems. Getting Photon Cannons can help you out immensely in the early game against Zerglings and if you feel that your foe means to press the advantage of a Zergling Rush, you'll want to get them as soon as possible to help support your Zealots. Shield Batteries can also be extremely useful when defending. If your opponent fails to pull off a Zergling Rush before you get your first Zealot, you shouldn't have too much to worry about in the early game. One Zealot can take 2-3 Zerglings by itself and once you begin Zealot production you should be ok. The only situation to watch out for in Zealot vs. Zergling encounters is overwhelming Zerglings numbers (more than 5:1 ratio of Zerglings to Zealots). This can easily happen if your Zerg foe builds multiple Hatcheries and builds only Zerglings and in such cases, you'll need to block up choke points with Zealots and Pylons and be well supported by Photon Cannons to fend off such numbers. When using a Zealot Rush to invade a Zerg base, take caution and keep an eye out for Sunken Colonies. The Zerg can produce Sunken Colonies as fast as Zerglings and only a few coupled with a few Zerglings can really damage a Zealot force. Hydralisks
Once the initial rushing is over, you can expect to face Hydralisks throughout the rest of the game. Hydralisks are relatively cheap and can also be produced in large numbers and they have a wide range of upgrades making them one of the most feared Zerg units, and rightly so. To counter them, you'll need Zealots to tie them up while you use Dragoons, Reavers or High Templar to finish them off. Reavers and Psionic Storm are your best bets against packs of Hydralisks and with a few good shots of either, you can raze a Hydralisk force in no time. Archons can also cause major damage to Hydralisks with their heavy hitting splash damage attacks. You can also use a Corsair's Disruption Web during a conflict to temporarily disable groups of Hydralisks. Handling Royal Pains
The Zerg Queen, if used well, can become a rather nasty thorn in the side for a Protoss force. With their Spawn Broodling ability eliminating your High Templar, Parasites to see what's happening at your bases and Ensnare to slow and reveal your forces, Queens should be hunted down on sight. Corsairs and Scouts are well suited for the task since they are faster than the Queen and relatively cheap. With a pack of 3 or more Corsairs or Scouts, you should be able to make every base a Queen-free area. Speaking of hunting, you should also make it habit to go Overlord hunting on a regular basis. The Overlord is even easier to pick off than the Queen and by killing stray Overlords (or even better, a stray pack of Overlords trying to hide in some corner of the map) you can slow down the Zerg unit production as well as decrease their chances of seeing or detecting any of your troop movements. Mutalisks
The Mutalisk is the most mobile Zerg unit and flying in packs, they can quickly find and eliminate your forces or expansions. Their "ricochet" attack also causes substantial damage to packed units or buildings when they're in large groups and can be quite intimidating. But don't worry, the Protoss have more than enough in their arsenal to take care of this problem. The first and best counter for Mutalisks is the Psionic Storm. Mutalisks will die beneath a Psionic Storm if they are left there for the entire duration of the effect even if they are at full health prior to the Storm. Also since Mutalisks stack on top of each other when grouped, making it even easier for you to destroy massive numbers of Mutalisks with a single use of Psionic Storm. But in most circumstances you cannot count on an opponent leaving their entire Mutalisk force in a Psionic Storm and they will usually escape with some of their health intact, so be sure to have support units nearby to quickly finish them off as they retreat. In addition to the Psionic Storm, the Protoss Corsair is a countermeasure that was specifically built to engage the Mutalisk. Their quick hitting splash damage Neutron Flare attack can dissipate hordes of Mutalisks with little effort and they are just as, if not more, mobile than Mutalisks so they can hunt down Mutalisks forces no matter where they go. For even better results, keep your Corsairs slightly separated when attacking to reduce the effect of the Mutalisk attack. Archons in groups are also deadly against Mutalisks. Their immense Plasma Shields protect them well from the Mutalisk attack and their splash damage attacks can kill Mutalisks in droves. However, if you do use Archons, be sure to have a few before engaging Mutalisks with them. A single Archon although powerful, cannot hope to prevail against a legion of Mutalisks. Guardians
The Zerg are usually fond of using Guardians to eliminate ground forces of any species and with their attack range and power, they have a good reason to. However, they have a few weaknesses you can exploit with your own units. For one, they cannot attack other aerial units, making it easy to force them to retreat with a few Corsairs, Scouts or Carriers. Also Psionic Storm can be cast over them outside of their attack range and with their slow movement, they usually fall prey to a well placed Psionic Storm. Even if you don't have either means of driving off a Guardian force, you can always use Shuttles to load your more powerful units (Reavers, High Templar) to safety before the Guardians can finish them off.