From Warhammer 40k - Lexicanum
"There is no peace among the stars, only an eternity of carnage and slaughter and the laughter of thirsting gods."
A dimension of pure energy used by mortals for faster-than-light-travel, the Warp is a dark reflection of the material universe. An ocean of chaotic, raw emotion given form, the warp is a dangerous place to travel through and come into contact with. Stirred by emotion and action, the Immaterium is the realm of Chaos, home to the dark gods and their followers.
Gods of Chaos
The mind of every sentient creature leaves an imprint within the Immaterium, and although the signature of one mind is almost insignificant, when the imprints of an entire race are combined they have a huge impact on the very nature of the Warp. When an emotion or faith grows strong enough, it becomes one of the denizens of the Warp and the strongest and most malevolent of these have become the Gods of Chaos.
Chaos Codices and Background Books
Chaos Space Marines
Ten thousand years ago the Imperium was racked by a terrible civil war called the Horus Heresy. Almost half of the Space Marine Legions of that time turned to Chaos and rebelled, plunging the Imperium into a terrible galaxy-wide war from which it has never fully recovered. Defeated, the Chaos Space Marines of the Traitor Legions, as they are now called, retreated to the Eye of Terror, where they lurk still, consumed by their hatred of the Emperor, the Imperium and all Mankind. Twisted by continual contact with the Chaos gods, Chaos Space Marines have become a terrible canker that eats at the heart of the Imperium and they might, one day, have their revenge...
The Forces of Chaos