Bunker

"We can take cover in these bunkers if things get too dicey."

- Terran Marine

The Terran Bunker is a defensive structure made of paristeel. It does nothing itself, but is made to be occupied by (up to) four Terran infantry units (Ghost, Marine or Firebat). These attack enemy units within sight and range of the bunker. The attack range of Ghosts, and perhaps other units, are enhanced within the Bunker (a Ghost's attack outranges that of a Guardian while within a Bunker). If a bunker is destroyed, the infantry units within appear in the center of the rubble and will continue to attack whatever they were attacking when the bunker was destroyed.

Uses
By far the most common use of Bunkers is defense or encampment. If a base is situated well, a few garrisoned  bunkers can easily defend it from a light to moderate attack. Even one bunker centrally located can prevent a single enemy unit from trashing buildings or spying (If accompanied by a Detector such as a Missile Turret). This is recommended if you have some Supply Depots clustered together.

As above, an essential supplement to a bunker is a detector. Siege Tanks and/or Goliaths work well emplaced behind them. Finally, if you know one of your opponents is Zerg, Firebats are highly recommended.

Weakness

 * It is weak to long range enemies like Guardians.
 * Without detectors it is weak to cloaked foes.
 * Large numbers of Zerglings might destroy it.

Trivia
Zerg units such as Infested Duran cannot enter the Bunker, due to a limitation of the StarCraft game engine.

StarCraft II
The Bunker in StarCraft II takes up a larger placement area (3x3 instead of 3x2, the same size as the StarCraft II Barracks and Factory) and has more hit points. Their carrying capacity seems to be unchanged.

Bunkers display how many units they have currently "loaded".