The Firebat is a Terran infantry unit that specializes in close-quarters combat.
Used predominantly during the Great and Brood Wars, they provided heavy support for groups of Marines with lighter armor. Very effective against small Zerg strains, Firebats were mostly used to prevent allies from being overwhelmed by swarm tactics.
After the Brood War, they have been gradually phased out in favor of the more hard hitting Marauders. However some remained in use, and a number were upgraded to Fire Drakes, a more moveable and ranged version.
For more information see Firebat on the parent site.