In the real time strategy computer game Starcraft®, the Zerg are a race of aliens that are a constant threat to all other species in the universe. The other races in the StarCraft® universe, the Terran and the Protoss, rely upon advanced technology to wage war upon one another; however, the Zerg have no technology. By assimilating other species into their gene pool, they mutate themselves to match whatever task is at hand.
The overwhelming majority of Zerg have no sense of individuality, but rather are controlled by the Overmind. This is a manifestation of their collective consciousness that directs them across the universe, causing them to assimilate any species with useful traits and destroy those which do not. The Overmind does not directly control all individual Zerg, but instead delegates this duty to self-aware generals called cerebrates. Although these cerebrates have individual thought, they are unable to defy the Overmind.
The Zerg do not build cities or settlements in the traditional sense, but rather grow them. All their structures are organic, with the exception of pre-existing buildings that they infest. As such, each Zerg colony is essentially one giant life form, which grows additional organs to make necessary genetic changes.
Each Zerg structure that is grown produces a bio-matter called the creep that covers the nearby area. The creep provides nourishment for the Zerg, and all of them recover from their wounds much faster when in contact with it. Some units have special abilities if they are standing on the creep.
All Zerg originally start out as larvae, which resemble armored, flat worms. These larvae contain all the genetic information necessary to mutate into whichever type of creature that is needed. If raw materials are needed, the larvae can mutate into drones, which will harvest any natural resources at hand, or mutate into Zerg structures.
This race uses many different creatures on the battlefield. At the bottom rung sits the Zergling, a small, voracious predator that is extremely dangerous in large numbers. The Hydralisk vaguely resembles a worm, and propels razor sharp spines at its enemies. The Ultralisk, a monstrously huge creature, utilizes razor sharp blades on its appendages to tear apart enemies of the Zerg.
The Zerg race in StarCraft® has been designed with the ability to create large numbers of units quickly and cheaply. As such, in real time strategy game slang, "Zerging" has become a term for overwhelming the opponent with a large number of cheap, relatively weak units. Such a strategy often works well only during the very beginning of a match.