Probe: This is the Protoss's worker unit. Not much to say, except they make good scouts. They're also useful because you only need 1 probe to start a new expansion and a good defense around it.
Zealot: This melee unit is the strongest of all the basic units. It's able to take almost anything down 1 on 1. However they're twice the cost of the other basic attack units. They're good all throughout the game, being able to run bases over late in the game. They're also very good economically, requiring no gas.
Dragoon: This is one of my favourite units. If you plan to use these units, you should definately research the range upgrade. With a decreased building time, they're very good for mass production, and can easily win games by themselves.
Shuttle: This is the best transport of the game, only after the speed upgrade. The Protoss shuttle, will be one of the fastest units in the game, making it very good for flying through heavy defense, or transferring troops. These also require no gas.
Reaver: This unit is now rarely used, due to the fact that drops from a shuttle can't be abused anymore. They also increased the building time for all the pieces you need for a reaver drop, making the reaver drop very slow. They still have plenty of firepower, and provide a good defensive threat against mass units as hydralisks, marines, and dragoons. However this unit is only good for catching people off guard, and I would not recommend using this unit, unless you can do serious damage. It was still very useful for sieging into a well defended base, as well as eliminating groups of enemy units.
Observer: These units must be created now since lurkers and dark templars are being used so frequently. I really don't like the fact that I need this unit just to run a base over. The speed upgrade is more useful as compared to the sight range. Personally I always liked these units, since it's like having black sheep wall on. =).
Dark Templar: These units are a huge threat to the Terrans, since it will force them to construct a engineering bay. They are expensivem so try to scout ahead of them if your plan to attack, prefferably with a probe, or dragoon. They make very good defensive units, forcing your opponent to bring in overlords and observers.
Dark Archons: These red puff balls, are useful for killing queens and wraiths. The maelstrom spell, isn't too great when you can cast psi storm, but it's an alternative and is still useful on masses of organic range units. The mind control spell, is great. However it's mainly used for defensive purposes, and that's not good. I wouldn't research this because I don't like to fool around, nor do I like to play defensive.
Templar: Hmm... I feel this is also one of the best units in the game. With psi storm and full mana, These guys can wipe out anything you want and are needed in every game you plan on winning. Not much else to say, except that you should learn how to use these guys and be able to click on the screen very fast.
Archon: You can create these units very early in the game, and they are very powerful. Archons do cause splash damage to the area they are attacking, making them slaughter mutalisks. They are kinda slow, and won't perform too well on small land areas. They also abosrb full damage from every attack, making that shield drop like no tomorrow. They make good anti-air units.
Scout: These units produce so slowly, and are so expensive that you should treat them as a capital ship. They aren't too useful, unless you have the speed upgrade. They are good, yet it's very hard to get a dozen of these ships. However once you do have a group, you can make these units do a quick hit and run attack on anything, destroying it, without losing a unit. Since these units are very expensive, you shouldn't waste them, use them mainly as a short task force against single units that become a problem like an overlord leering around your base or a zergling just off your screen bugging you =).
Corsair: These units make good mutalisk and wraith killers. They should be produced only for that purpose, and disruption web spell. This spell is very powerful, and can wipe out a base if used correctly. However this takes a lot of micromangagement. This should best be used, once you have already attacked the enemy, and there units are stationary. NEVER take a small or somtimes even large groups of these up against a Battlecruiser since it most likely has Valkryie support which can decimate these liitle guys to zergling feed quickly.
Carrier: This blimp now as an impressive starting armor of 4. They are however very expensive, when you include all the upgrades and interceptors, making these units rare on the battlefield. They're the last resort for breaking into a base. In B.W. they are now rarely used in masses because of Valykries and Corsairs chewing them to pieces, use them VERY wisely.
Arbiter: Arbiters die very fast since they're usually the only unit the opponent can see. They aren't very useful, to attack with, or even support an attack. However they are great transports, being able to take hits, and recall a huge force. They also make good defensive units since they can cloak a base full of units from enemy site, and being able to cast stasis on any group of units. This can cut a group of 12 battle cruisers, to only 4-5 which you can destroy easily. With the new cost, they are affordable, yet they aren't useful unless you have an army to attack with.
