Once the proud elite warriors of the God-Emperor of mankind, now the followers of the Chaos gods.
During the time of the Horus Heresy, set roughly 10,000 years before the events of Warhammer 40k, fully half of the legions of the Space Marines were turned to Chaos.
Led by Abaddon during his Black Crusades, twisted and warped by the magics of the immaterium, they now seek their vengeance on the Imperium they believe betrayed them, looking to cast down the False Emperor and his followers.
Chaos Space Marines (or Heretic Astartes) are a diverse army, being able to draw specialist troops dedicated to one of the four Chaos gods: Khorne, Nurgle, Tzeentch, and Slaanesh. There are also a number of units which are 'undivided', and owe their allegiance to no god in particular, such as the long-ranged Havocs, or the jump-pack using shock troops known as Raptors.
Being part of the ruinous powers of Chaos has its advantages in the game too, with Chaos Space Marines being able to ally with the Chaos Daemons, summoning them mid-game through sorcerous magics.
Combined with the variety of models available, and the customisation options which come with being able to create your own chapter of heretics with their own colour scheme and backstory, this makes the Chaos Space Marines a great army both to paint and to play with. Plus, it's always fun playing as the 'bad guys' (it's all about perspective!)