Terran
===Command Center=== The command center is the Terrans' primary building. It produces the worker SCVs, and is where gathered resources are dropped off. It can sport two fancy extensions, the Com-sat station, and the Nuclear silo. Also, the CC (Command Center) can lift off and be moved to a new location. This is useful in that you can build an CC for a nearby expansion somewhere safe in your main base, and then fly it to its new location (however you'll leave any expansions you've built behind). A CC costs 400 ore.
===Comsat Station=== The comsat center is an upgrade for the CC, it costs 50 ore, 50 vespene gas to construct. It can only be constructed once the Academy has been built. When energy allows, you can use the comsat station's scanner sweep to reveal any portion of the map for a short time (~30 seconds). The perfect recon tool. I recommend having two of your CC's with this add-on, and the Comsat's bounded to your 0 and 9 quick keys. The scanner sweep also detects cloaked units.
===Nuclear Silo=== The nuclear silo is the other upgrade for the CC, you can only have one upgrade, so it's either the Comsat or this. A nuclear silo costs 50 ore, 50 vespene, but to build a nuke for it will cost another 200 ore and 200 vespene gas. To actually make use of the nuke you'll need a Ghost to target where the Nuke should land. Nukes are of only limited use: if the ghost that's targeting the nuke dies before the nuke has found its target, then you don't get an earth shattering kaboom.
===Supply Depot=== Supply depots generate the nessesary supplies to feed and maintain your troops. Don't worry though, you don't have to set supply lines or anything like that; all you need to do is have enough supply depots to support your troops (otherwise you can't build new troops). A decrease in Supply will not cause a decrease in your troops' abilities or numbers. A supply depot generates 8 supply, which can be used to construct new units.
===Refinery=== The refinery can only be built on a vespene geyser. It costs 100 ore. Once built, SCVs can enter the refinery and bring back valuable vespene gas.
===Barracks=== The barracks is the first structure you can built that produces attack units. It costs 150 ore. It can produces Marines, Firebats and Ghosts. If you're running the expansion pack "Brood War" it can also produce medics. The barracks can lift off and be moved.
===Bunker=== The bunker is a defensive structure that can be loaded with infantry (and SCVs). Units in the bunker cannot be damaged until the bunker is destroyed, however they can still fire out.
===Academy=== The academy is a research structure: it offers Stim Packs and Enhanced Marine range upgrades. In broodwar it also offers a variety of medic upgrades. The academy must be built for medics, firebats and ghosts to be produced. It cost 150 ore, and 50 vespene (I think).
===Engineering Bay=== The engineering bay is a research structure that provides infantry weapon and armour upgrades. It costs 125 ore, and can lift off. It is also required to build Missle Turrents.
===Missile Turret=== The missile turret is a ground to air defense structure. They cost 100 ore apiece. They are worth having sprinkled around your base because they are "detectors". And they discourage opponents from dropping their units off in your base. It's much easier to kill an overlord and having the two Ultralisks its carrying plummet to their deaths than to permit the dropoff and kill the Ultralisks. Besides that, they just look cool with their spinning payload...
===Factory=== One of the Terrans' most important structures, the Factory produces the Siege Tank, as well as Vultures and Goliaths. The Factory costs 200 ore and 75 vespene, and can get the Machine Shop add-on. The Factory can lift off, but it will leave the Machine Shop behind; it can also connect to idle Machine Shops.
===Machine Shop=== The machine shop add-on provides upgrades to units produced in the factory. It's built by the factory for 50 ore and 50 vespene. You'll want this to get those Siege Tanks. Also the Siege upgrade is available here, as well as Spider Mines, and you can also buy a Goliath upgrade that extends the range of their anti-air missiles.
===Armory=== The armoury provides weapon and armour upgrades for vehicles produced by the Factory and Starport.
===Starport=== The Starport produces your flying units. Wraiths, Dropships, BattleCruisers, and Science Vessels (and Valkyries in the BroodWar Expansion). It costs 150 ore, 100 vespene. It can lift off but in doing so loses any Control Tower expansions it has built.
===Control Tower=== The Control Tower is required to build most of the Starport's units (only the Wraith can be built without it). It allows you to research Wraith Cloaking and an energy upgrade for the Wraith's engines.
===Science Facility=== The science Facility is the last research structure the Terrans can build. I think it costs 100 ore and 150 vespene. It can lift off, but by doing so it will leave behind whichever expansion it has. It researches a number of abilities that the Science Vessel can use.
===Physics Labratory=== The physics labratory is an add-on for the Science Facility. It allows you to research the Yamato Cannon. You must have a Science Facility with attached Physics lab to construct Battle Cruisers.
===Convert Ops Station=== The convert ops station allows you to build Ghosts and Nukes. It also provides a number of options for Ghost research. Cloaking is a good idea if you plan to nuke anyone. Lock down is a wonderful tool for disabling enemy mechanical units. Watch the Protoss player cry when you lock down his 12 Carriers, and then blow them up with your 'rhines' --- or anything for that matter
Protoss
===Nexus=== The Nexus is the Protoss' primary building. It produces workers, and is where gathered resources are dropped off. A Nexus costs 400 minerals.
Zerg
===Hatchery=== The Hatchery is the Zerg's primary building. It is where gathered resources are dropped off. Unlike the other teams, the hatchery is where all zerg units are built. The Hatchery also allows zerg to evolve to the next 'age'. A Hatchery costs 350 minerals.