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. Alongside the protoss and terrans, the zerg stand as one of the three dominant species of the Milky Way and are universally feared, hated and hunted as such. The Swarm in itself makes up a third of the power of the Koprulu Sector.
For more information see Zerg Swarm on the parent site.