Nexus: This is the Protoss Nexus... nuthin much to say except that produces probes for you.
Pylon: These are your supply structures and the power source for your other structures. Basically you want to get a good area around your base, with your intial set of pylons. This is only to make sure you can warp in cannons around your base. Also you should double up on pylons to make it harder to shut down a structure.
Assimilator: This isn't all too exciting either, basically warp it in, and wait till it finishes.
Gateway: These produce the most important units of the game, making these very important. You should be able to support 2-3 gateways per base.
Cybernetics Core: This is a very important structure since it leads to the production of dragoons, and the warping of advanced structures. You don't want to lose this building. Also during the time you decide to warp this building in, can you risk being attacked, since you had to sacrifice two zealots for this structure.
Forge: This is another important stucture because it leads to the production of photon cannons, and that's a good thing.
Photon Cannon: These are the best defensive structures out of all three races. Being a detector, who can attack air and land, with 20 normal damage, and at a good range and attack rate makes them worth the money. Use these sparsely and sparingly though, because they aren't cheap, and aren't mobile like a dragoon. There main purpose is to keep out dropships, cloaked units, and queens from entering the base. However you can use them offensively, though it's not worth it if your opponent can still gather gas.
Shield Battery: These have a longer range after the patch. These are very good for extending the life of your dragoons and reavers who take a lot of abuse. You should build a few of them since 1 battery shield can't support too many units. They make good structures to use offensively, where you can recharge your units after hitting a base. Try building a few outside of an opponents base.
Robotics: This is the most important structure on an island map. It's generally used to produce shuttles, and observers. Reavers still can be used to raid a worker line, yet they won't be effective since it's not worth it to pick up and drop again. Basically reavers will be a hit and run tactic.
Robotics Support Bay: This is where you gain access to the reaver as well as the shuttle upgrade. The reaver upgrades aren't worth it. However the shuttle upgrade is definately worth researching. < p>Observatory: This leads to the production of observers, which you'll be needing now with all the cloaked stuff out there. Research the speed upgrade, since you get to cover more ground faster, making it unnecessary to build more observers. They are expensive so try not to lose them.
Citadel of Adun: This structure has no purpose except to make your zealots stronger than they already are. The speed upgrade is needed to equalize against the range upgrade of the other units.
Templar Archives: After this structure is warped in, you get access to 2/2 upgrades. Besides that, you gain access to dark templars and templars. Both are very useful, yet very expensive, usually making you choose which to produce since you can't produce both. Dark templars are good against Terrans, and on maps that are mostly land. However templars gives the ability to wipe out huge forces, and make better raiders on mineral sites.
Stargate: All of your air force comes from these structures. Not much to say, except if you can support them, then build more.
Fleet Beacon: This is a necessary structure, and a very expensive one too, if you plan to use scouts, corsairs, or carriers. The upgrades here are essential, since most of units produced of the stargate aren't useful.
Arbiter Tribunal: Well you'll want to create this if you plan to use arbiters. They are expensive still, so if you do decide to use them, make sure that your attack will be successful by recalling everything you have.
