Zerg are completely different from the Terran and the Protoss races.  They are formed from different species, integrated into swarm through Zerg infestation.  They are essentially a squirming mass of living networks.

These creatures are an insect-like species, which have rapidly and selectively evolved, and will stop at nothing to achieve absolute domination.  The Zerg are completely organic and have no technology to create Armor, Starships or Weapons.  Instead everything they represent and utilize are efficiently fulfilled through biological adaptation and mutation of Zerg strains.  All the Zerg buildings are organs within the living and a growing Zerg nest.

While each race fights against one another, Zerg certainly are the bad guys of the Starcraft Series.  They want to dominate with utter supremacy in every part of the universe.  Their ravenous nature means they will stop at nothing to over-run Protoss and Terran forces.

Zerg Build Orders