Source: CVG
_____________
In 1998, StarCraft told the story of three races in conflict, and it went a little something like this... The Terrans - human colonists - are torn apart by civil war and find themselves stuck in the middle of a war between the Protoss and the Zerg. The Protoss and the Zerg are enhanced alien races given an evolutionary leg-up by a meddling super-race called the Xel'Naga.
The Protoss gained psionic powers that singularly failed to stop them from waging civil war, while the Zerg were given advanced parasitic powers, turning them into a hive-like plague, wiping out or infesting everything in their path.