Zerg
The Zerg is an insectoid race, that is incapable of disobeying orders and is controlled by a Cerebrate. Several types of creatures pertain to this race which has a different way of constructing buildings: the Drone morphs into the structure and constructs it into a short amount of time. They biologically evolve and carry a blind devotion to the Celebrate, that responds to the ruler Overmind, thus making them a reputable force.
The Zerg were first discovered under a less evolved formed on the unstable planet of Zerus by the ancient Xel'Naga civilization. The ancestral Zerg creatures were manipulated genetically to survive the firestorms on their planet and thrive. They later began living as parasite organisms, assimilating the genetic processes of these host animals. The Xel'Naga then decided to create the Overmind, a brain-entity that would control the Zerg creatures.
The Zergs slowly invaded Terran worlds, evolving due to exposure to vespene gas. The Overmind infested the surface of the Terran Confederacy colony then the Terrans were slowly defeated and enslaved. The Terrans did not warn other worlds about the invasion, giving the Zergs the chance to attack Mar Sara unopposed, when the StarCraft actually begins.
Strengths:
Fast attack force, can heal their structures and themselves, can increase the amount of health the Hatchery has
Weaknesses:
Takes long to upgrade, their drones morph in to the building so more drones gives you the chance to move better into the game.