StarCraft/Zerg buildings

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 minerals.

Comsat Station
The comsat center is an upgrade for the CC, it costs 50 minerals, 50 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 and all units (Including cloaked and burrowed ones) for a short time (~15 seconds). The perfect recon tool. Comsats can be "hot keyed" so a cloaked unit can be revealed in the heat of battle and killed before causing too much trouble.
 * Prerequisite: Academy

Nuclear Silo
The nuclear silo is the other upgrade for the CC, you can only have one upgrade, however it is allowed to swap back and forth between them by lifting off and settling down next to the other one. A nuclear silo costs 50 minerals, 50 vespene, but to build a nuke for it will cost another 200 each. 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 boom.
 * Prerequisite: Science Facility with attached Covert Ops

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, but must be rectified before new units can be ordered. A supply depot generates 8 "supply", which is the cost of the largest units (Battlecruiser and Nuke).

Refinery
The refinery can only be built on a vespene geyser. It costs 100 minerals, but is a near absolute necessity for anything more then marines and SCVs. All air units require vespene. Once built, SCVs can enter the refinery and bring back the valuable vespene gas.

Barracks
The barracks is the first structure you can build that produces attack units. It costs 150 minerals. It can produce 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.
 * Prerequisite: Command Center

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.
 * Prerequisite: Barracks

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 minerals.
 * Prerequisite: Barracks

Engineering Bay
The engineering bay is a research structure that provides infantry weapon and armour upgrades. It costs 125 minerals, and can lift off. It is also required to build Missile Turrets.
 * Prerequisite: Command Center

Missile Turret
The missile turret is a ground to air defense structure. They cost 75 minerals 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 spinning endlessly... More seriously, a endless line of turrets occupying a strategic bluff or pass (Hopefully backed up with ground support units) can have a psycological effect on a human opponent.
 * Prerequisite: Engineering Bay

Factory
One of the Terrans' most important structures, the Factory produces the Siege Tank, as well as Vultures and Goliaths. The Factory costs 200 minerals and 100 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.
 * Prerequisite: Barracks

Machine Shop
The machine shop add-on provides upgrades to units produced in the factory. It's built by the factory for 50 minerals 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.
 * Prerequisite: Factory

Armory
The armoury provides weapon and armour upgrades for vehicles produced by the Factory and Starport. It allows for the production of Goliaths from the Factory and Valkyries form the Starport.
 * Prerequisite: Factory

Starport
The Starport produces your flying units. Wraiths, Dropships, BattleCruisers, and Science Vessels (and Valkyries in the BroodWar Expansion). It costs 150 minerals, 100 vespene. It can lift off but in doing so loses any Control Tower expansions it has built.
 * Prerequisite: Factory

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 which gives longer cloaking time.
 * Prerequisite: Starport

Science Facility
The science Facility is the last research structure the Terrans can build. I think it costs 100 minerals 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. It also allows advanced weapons and armour upgrades for both infantry and mechanical units.
 * Prerequisite: Starport

Physics Lab
The physics laboratory is an add-on for the Science Facility. It allows you to research the Yamato Cannon for battlecruisers. You must have a Science Facility with attached Physics lab to construct them.
 * Prerequisite: Science Facility

Covert Ops
The covert 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 anything on hand.
 * Prerequisite: Science Facility

Nexus
The Nexus is the Protoss' primary building. It produces workers, and is where gathered resources are dropped off. A Nexus costs 400 minerals.

Assimilator
The Assimilator must be built on a Vespene Geyser before the Protoss workers can collect gas. It costs 100 minerals.

Pylon
The Pylon is the power source for all Protoss buildings (except for the Nexus and Assimilator). Protoss buildings require at least one Pylon built nearby in order to function properly. If a Pylon powering a building is destroyed, the building will stop working. A Pylon costs 100 minerals.

Gateway
The Gateway is where Protoss ground units (Zealot, Dragoon, High Templar, and Dark Templar) come out. It costs 150 minerals.

Forge
The Forge is used for upgrading Ground Attack, Ground Armor, and Shield Armor. It costs 150 minerals.

Shield Battery
The Shield Battery can be used by all Protoss units to recharge their shields. It costs 100 minerals.

Cybernetics Core
The Cybernetics Core is used for upgrading Air Attack, Air Armor, and increased attack range for Protoss Dragoons. It costs 200 minerals.

Photon Cannon
The Photon Cannon is a defensive structure for the Protoss. Its ranged attack can hit both ground and air units. It can also detect cloaked units. It costs 150 minerals.

Hatchery / Lair / Hive
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 evolution to the next 'age', giving new possibilities for buildings/units. The first evolution changes it to a Lair, the 2nd to a "Hive". A Hatchery costs 300 minerals.

Spawning Pool
The spawning pool is an early structure requiring 200 minerals to construct. It allows allows the player to create Zerglings and has two upgrade options for Zerglings. The second upgrade cannot be completed until the Hatchery has been evolved into a Hive.

Hydralisk Den
The Hydralisk Den as its name suggests allows the production and support of Hydralisks. Dedicated like most zerg buildings to one unit or unit type it allows two upgrades in StarCraft and a third with the Brood Wars expansion. The basic two upgrades that it allows are an increase in speed and an increase in range. Both quite useful as the Hydralisk is Considered by most to be the backbone unit of the Zerg forces. The Expansion upgrade is the Lurker upgrade which allows the Hydralisk to go back into its egg and reemerge as a lurker.

Evolution Chamber
This building is where the Zerg ground unit upgrades can be found.

Spire
One of the more important Zerg structures. Allows spawning of air units at the hatchery. Also air unit weapons and armor can be upgraded here.