>>10290222The Imperium of Man is ruled over by the God Emperor of Mankind, who is virtually dead, and he is kept 'alive' by some groovy voodoo on a special throne. His psychic energy is used to power a beacon that is used as a reference point for human travel through the Warp.
The Warp is a sort of parallel dimension of evil and chaos (Think where Event Horizon went when she was lost for a decade), where the Chaos Lords reside. They are constantly trying to spill through to our universe, and spread their chaos. At their disposal are thousands upon thousands of demons and corrupted Space Marines (The elite soldiers of the Imperium).
Orks are an infestation that pop up hear and there, have a good Waaagh, and get the shit knocked out of them, and then they pop up somewhere else.
The Eldar are space elves, and generally hate all other races.
The Tau are a reasonable, democratic race who are rapidly advancing, technologically.
The Necrons are the sort of zombie robot soldiers of an older, more evil 'god' than the Chaos gods.
The Tyranids are Zergs (It's actually what Zergs were copied off of). They're spreading from somewhere outside our galaxy, and slowly devouring everything that is known with their seemingly unstoppable swarm.
Oh, also, the official state religion of the Imperium is the worship of the God Emperor. Heretics(Virtually anything can be heresy) and Xenos are killed, mercilessly. Entire planets have been irreparably annihilated, utterly extinguishing all life, or future hope of life, on the surface, for the crime of heresy.