Chaos

Ten thousand years ago during the galaxy-shattering wars of the Horus Heresy fully half the Space Marine armies of that time became corrupted by Chaos and rebelled against the Emperor. Led Warmaster Horus, the Legions of Chaos almost succeeded in wresting control of the galaxy from the Emperor. They did not succeed, but the wounds the Emperor suffered in combat against the Warmastter drove him to the Golden Throne and ended his waking life.

With the defeat of the Legions of Chaos the vanquished Space Marines fled into the Eye of Terror. There, close to the intense energies of the warp, they took worlds for their own, becoming the lords and masters of daemon realms under the patronage of their chosen gods. From these infernal regions the Chaos Space Marines continue to launch their wars of vengeance against the Imperium. Within their warped realms time has been abolished, so that the very same Space Marines warriors who fought against the Emperor ten thousand years ago still live and continue to make war against the Imperium today. For them the strands of time have become interwoven so that the past, present and future have merged into one endless, eternal battle.

-excerpt from the second edition Chaos Codex

The Chaos Marines are pretty well similar to the normal Space Marines except that their technology is worse. To compensate for that, they have the support of Daemons, fearsome beings that come from the warp. These beings are summoned during the battle and can give an unexpected turn in the tides of battle. Be careful when you fight Chaos, because as the name implies, they can do some unexpected stuff.