The Zerg where in the distant past a small larvae-like animal living on the planet Zeerus. When the Xel Naga, a mighty alien race arrived on Zeerus, they experimented with the Zerg, in order to make a race of one mind, a race of psychic perfection. However, the Zerg not only developed a collective mind (later called "Overmind"), the began to assimilate other life forms on their world into the Zerg brood, by taking their DNA and hatching a Zerg form of the animals by their eggs. Those not selected for assimilation where exterminated, and soon Zeerus was overrun with Zerg! Even the mighty Xel-Nagas where felled by their creation, and the Zerg set out to pollute the universe with their abomidable genetics!
The Zerg swarm, with the exception of one infested Terran (Kerrigan), are under the complete control of the Overmind, which can manifest itself as a giant mountainous blob of grotesque flesh. Next in rank to the Overmind are the Cerberates, giant brain-beings that each control a brood, a different colored branch of the Zerg swarm. These broods are named surprisingly after earth mythological characters (the Jormungand Brood, Fenris Brood, Garm Brood, etc), perhaps indicating a previous connection with Earth.
In the 26th century, in a region of space known as the Koprulu Sector, the Zerg encountered two powerful races: the war-like Terrans (or humans), and the psychic Protoss, an imperial race that was also experimented on by the Xel Naga. The Zerg fought both races for control of the region, and planned to assimilate the Protoss, and perhaps even the Terrans as well, into their horrendous Hive.
Now the Overmind's plans are being set in motion, as a new form of Zerg, a infested psychic terran named Kerrigan, has emerged from her chrysalis, and the battle for dominance in the koprulu sector reaches fever pitch...