The Zerg Swarm are the primary villain faction the RTS game Starcraft. They are very similar to the Tyranids from Warhammer 40,000, being a collection of assimilated, genetically enhanced species controlled by a hive mind, which seeks to assimilate new species. The similarities are because Starcraft was originally planned to be Warhammer 40,000 game, but Blizzard, the company that made Starcraft, lost the rights to use Warhammer. The plot depicts them as the most powerful faction because of a massive numerical advantage over their enemies. The size of swarm, and exactly how strong the Zerg units are plotwise is highly ambiguous, since they're from a video game.
Gameplaywise, the Zerg focus on fast and inexpensive units. Starcraft's popularity has resulted in people labeling fast, cheap, basic units in other RTS games as Zerglings (named after a Zerg unit that fits that description).