The Zerg are a terrifying and ruthless amalgamation of biologically advanced, arthropodal aliens. Dedicated to the pursuit of genetic perfection, the Zerg relentlessly hunt down and assimilate advanced species across the galaxy, incorporating useful genetic code into their own, and exterminate any species that might corrupt the purity of the Zerg genome.
The Zerg were "created" by the Xel'naga on Zerus. The Xel'Naga were searching for a counterpart to merge with the Protoss, gifted with the purity of form, and continue their lifecycle. In the Zerg they found a worthy candidate for the purity of essence. To prevent a similar fragmentation and regression experienced by the Protoss during the Aeon of Strife, the Xel'Naga subordinated all the Zerg to a single entity: the Overmind. However, the Overmind fell under the control of a being known as the Dark Voice. The ancient species attempted to keep their presence a secret from the Swarm, but eventually the Overmind became aware of them and ordered the Zerg to attack. The Xel'Naga were driven from Zerus, and the Zerg gained knowledge of the Protoss. The Dark Voice's control forced the Overmind to seek out the Protoss. The Overmind believed assimilating the Protoss would make the Zerg "perfect", and directed the Swarm into space in search of Aiur, the Protoss homeworld. Along the way it assimilated multiple species in preparation for the confrontation. As the Swarm neared the Koprulu Sector and the volume of space under Protoss stewardship, they discovered the Terran. Of the species encountered, the Terrans were unique in having psionic potential. With such abilities the Zerg would be able to combat the Protoss on even terms.
The Swarm started the Great War by invading the Terran worlds, but were met by a Protoss counter-attack. The Protoss were aware of the Zerg's Xel'Naga origins and, seeing the effects of Zerg infestation, resolved to prevent any Zerg foothold. Protoss Executor Tassadar, leading the Koprulu Expeditionary Force, was reluctantly forced to purify numerous Terran worlds. Even so, the Zerg were not stopped.
The Great War also sparked a large scale Terran civil war. Terrans on both sides of the civil war sought to use the Zerg for their own advantage. The rebel Sons of Korhal manipulated the Zerg through psi-emitters, drawing the Swarm onto to the defenses of the failing Terran Confederacy first at Antiga Prime and then at the Confederate capital world of Tarsonis. At Tarsonis, the Zerg not only delivered the killing blow to the Confederacy, but also captured Sarah Kerrigan, a psionic ghost operative. The Overmind believed Kerrigan was the key to stopping the Dark Voice. The Swarm withdrew to Char to complete the infestation of Kerrigan into the Queen of Blades. She was the Overmind's way to resist the Dark Voice's indirect control of the Swarm.
Kerrigan's presence drew the Protoss leaders Tassadar and Zeratul to Char. The latter was a Dark Templar with the ability to permanently slay cerebrates. Striking an alliance, the two Protoss groups fought repeated battles against Zerg forces led by Infested Kerrigan. Zeratul slew Zasz, inadvertently giving the Overmind the location of Aiur. The majority of the Zerg Swarm promptly traveled to Aiur, leaving Kerrigan behind with a small number of Zerg to fight the Protoss still on Char. The Zerg on Aiur enjoyed victories over the Protoss.
The Protoss on Char eventually escaped to Aiur, where they defeated the Overmind.
The death of the Overmind caused the temporarily leaderless Zerg to rampage; most of Aiur's population was killed. The Zerg split into two factions, one led by Daggoth (and the second Overmind) and the latter led by Infested Kerrigan. The Protoss fled to Shakuras followed by Daggoth's Zerg. The Protoss collected a pair of artifacts in order to destroy the Zerg and make their new home safe, while forming an unexpected alliance with Kerrigan. The Protoss destroyed the Zerg on Shakuras in order to protect themselves, belatedly realizing that Kerrigan was manipulating them into fighting on her side in the Zerg civil war.
The arrival of the United Earth Directorate in the Koprulu Sector upset Kerrigan's plans. After conquering the Terran Dominion and chasing after its emperor, Arcturus Mengsk, they took control of and activated the powerful anti-Zerg weapon, the Psi Disrupter, despite interference from Kerrigan's enigmatic minion, Samir Duran. Using other technologies, the UED took control of the second Overmind, turning most Zerg to their side, while the Disrupter weakened Kerrigan's control over her own forces. Kerrigan was left in a weak position and resolved to strengthen her side by striking an uneasy alliance with Jim Raynor and Arcturus Mengsk. After the alliance severely weakened the UED Kerrigan betrayed them, then she turned her attention to the Protoss. With assistance from Samir Duran she kidnapped the Dark Templar Raszagal and forced Zeratul to slay the second Overmind for her. Now in charge of all the Zerg broods, she was suddenly abandoned by Duran.
After the Brood War the Zerg remained on worlds they had already infested, sending out forces to explore and exploit various worlds and assimilate new species, and refrained from engaging in overt hostile actions against the Terrans and Protoss for four years. No expeditions sent to Swarm-controlled worlds have come back alive.
The Swarm started the Second Great War by attacking Terran space in search of a Xel'Naga device. The Zerg were beaten to the device by Raynor's Raiders. Char was invaded by a Terran battlegroup that used the device to de-infest Kerrigan.
Unique aspects of the Zerg units include
- Creep (a terrain effect which is required for all Zerg structures to be built upon (with exception of Nydus Worm and Extractor)
- Hatchery - Spawn Larvae (a buff which causes the Hatchery to create a larva every 15 seconds with a maximum of 3)
- Queen - Spawn Larvae (a targeted ability which causes a Hatchery to spawn 4 extra Larvae in 30 seconds, with a maximum of 19)
- Spine Crawler - Root/Unroot (a morph ability which allows the building to move and relocate itself)
- All Units - Burrow (a morph ability which transforms units into their respective burrowed counterparts)
http://starcraft.wikia.com/wiki/Zerg for most of the text.