Imperium: A nightmare society based on ignorance, fear, control. Every horrible "means-justify-the-ends" trope you can think of put in one place, and run on religion and a horribly bloated bureaucracy, which is in turn run by a secretive society of people who have monopoly on all technology, partially because they halt all independent technological progress in the name of religious dogma.
Chaos: The negative emotions of all man, given form, manifesting it's self though endless hordes of hateful deamons, lovecraftian horrors and the pissed cultists, big and small, who worship them. Has a lot of converts who were too damn mean for the above group, so that is a mild indicator of the niceness of these guys.
Orks: An ubiquitous warrior race that can survive anywhere, make weapons from anything, and live only to fight because they think it's fun. Though their eyes, it's pretty funny. Though everyone else's eyes, it sucks balls.
Eldar/dark eldar: a faction of arrogant stuck-up bastards who killed half the galaxy by partying too much, and now are willing to kill anyone else on a whim because they see everyone else as animals standing in the way of the restoration of their empire. The dark eldar are the ones who are still busy partying and put the emphasis more on the "kill you on a whim" part, except they rape, mutilate, and torture you first for kicks.