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...
The following are the different forms the Zerg take (this is not an exhaustive list)
1. Zerg Larvae
Zerg Larvae are the Zerg that most resembles the original animals of Zeerus. From these beings can be forged an egg with enough crystals mined and provided for them. They are in and of themselves harmless, yet the Zerg swarm is dependent on them.
Drones are the worker caste of the Zerg Swarm. They are miners, workin on crystal deposits and Vespene Gas Geyers to build Zerg armies and fortresses. Drones are also capable of mutating into living structures, such as Zerg Hives, Ultralisk Caverns, both spore and sunken colonies (defensive structures), and many more.
Overlords are the controllers of the swarm. Though the Cerberates, and of course the Overmind, are the swarm's masters, they cannot control and manipulate hordes of Zerg without Overlords. Their thunderous, cacophonic vocalizations are the stuff of Legend. They watch over their siblings, floating above like German Zepplins of old. They cannot defend themselves, yet their existence is their defense, for without enough Overlords, no new Zerg can be produced.
Zerglings are the backbone of the Zerg ground forces. From one Larvae egg can erupt two Zerglings, and they are fast, feral, and lethal. Their beserker tactics and swarm attacks are the stuff of Space legend, and if enough are produced at the beginning of a battle, they can easily overrun human and Protoss command bases.
The most recognizable and iconic Zerg, these monsters where original plant eating animals called Slothiens. Some of their herbivorous teeth are still recognizable, though it is not recommended that you get close enough to see for yourself: these animals are formidable acid spitters, capable of hitting troops on the ground and aircraft above. The S to air acid spits have a far greater range than their ground attacks, and though the animals are armed with blades that can cut men in two, they are rarely used.
Recently the Zerg Hydralisk has been mutated, to where it can change into a "Lurker" a gigantic Hydralisk that when burrowed can attack with a stream of spines that erupt from the ground. This weapon is especially devistating, turning Marines into mince meat in the blink of an eye.
6. Zerg Ultralisk
The Ultralisk is the Zerg Bio-tank. Over a hundred feet tall an thousands of tons, these animals can withstand numerous blasts from 26th century human artillery and keep fighting. The older forms had but 2 claws, but recently four are found on the evolved Ultralisks.
These monsters are terrible to behold and are best attacked from the air.
7. Zerg Mutalisk
Mutalisks are the pride of the Overmind's airforces. The perfect attack beast, Mutalisks can devistate fortresses of any type when massed in incredible numbers (which the Zerg can easily do). Their speed, fear-inspiring battle cry, and acid vomits are so formidable that the Terrans have developed the Valkrie missile ship, among the most advanced air-to-air attack craft known, to deal with the threat.
Mutalisks can be evolved quickly into two forms:
a. The Guardian
The Guardians are demolishers, mighty Air-to-Surface attack animals that, if controlled and used today, would be the perfect beast of burden for demolition crews. Their mighty acid balls inflict far more damage than the normal Mutalisk acid vomit, and their shells are thick and durable, capable of withstanding multiple missile attacks and laser fire. If you see one of these approaching, RUN!
The Devourer is designed to attack only aircraft, but it does this superbly well. Its tremendously powerful acid spits corrode armored metal like a sumo wrestler devours chucky cheese pizzas. Valkries and Protoss Scouts find these animals to be formidable opponents. When grouped in great numebers alongside a large number of Guardians, these animals become part of an almost unstoppable Airfleet.
8. Zerg Queen
Zerg Queens are among the most feared members of the Swarm. These flying hockey moms of the cosmos can send parasites into any life forms, in order to see what they see and aid the Overmind and its Cerberate lieutenants to plan attacks or sieges. Sometimes those humans or Protoss who are infected are subsequently killed to keep from being used as innocent spies, yet this is not the worst the Queens are capable of.
Zerg Queens are also capable of infecting lifeforms of a vast size range (from Terran to Ultralisk) with Broodlings, tiny Zergs that violently erupt from their hosts.
Broodlings, once "hatched" ca be used for attack and defense, yet they are not as powerful/armored as Zerglings and eventually burst and die without need of being attacked.
Zerg Queens can also infest Terran command centers, by shooting a parasite into one. The parasite takes some time to work, but by the time it is finished the command center is alive, giving birth to infested humans.
Infested Terrans are walking bombs, capable of exploding upon contact with their targets. They have been known to destroy AFVs, Tanks, even bunkers with one blast. These nightmarish fiends are the future of humanity if the Zerg ever fully assimilate their genetics into the Overmind and his swarm.
9. Zerg Defiler
These fiends are the bane of Terrans and Protoss. Unable to defend themselves physically, these brutes can nevertheless launch bio-weapon attacks, such as Zerg clouds (blind enemy troops but not friendly Zerg and allies) and a red pollution which renders armor useless and leaves even mighty battleships easy prey for far weaker Zerg.