DEATH TO THE FALSE EMPEROR!
In Warhammer 40K, the Chaos Space Marines are the name given to the legions of Imperium Marines who turned their back on the Emperor and Humanity in favour of Horus and the Chaos Gods during the Horus Heresy. This numbered to about half the Imperial army at that time, and with constant recruits into their ranks, they are the Imperium's true match.
The Chaos Marines believed Horus, was who should be the leader of humanity, not the Emperor. They have vowed to destroy the Imperium in vengeance of Horus's death by the hands of the Mankind's Emperor. They now roam in the Eye of Terror, the greatest warpstorm in the galaxy created by the fall of the Eldar and Slaanesh, one of the four chaos gods leading the traitor army. They sit and wait in hope for a chance of revenge against the Imperium, mankind and the mortally-wounded Emperor, who is kept alive by the Golden Throne after the battle with Horus.
In Warhammer 40K, the Chaos Space Marines are split into four parts, each that worships a different god. The different gods are:
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Khorne:
Khorne is the god of war, or the blood god. He embodies war, conflict and death. He is kept alive by the constant deaths in the Warhammer 40K universe, and sits on a brass throne surrounded by the skulls of those who have died in his name. If a Warhammer 40K commander were to choose a Khorne army, they would have choice over the blood-thirsty Khorne Berserkers, Blood Hounds and Blood Letters, and the almighty Greater Daemon, the Bloodthirster . Each of these embodies the bloodlust of Khorne, and are all fine-tuned for close-combat.
The powerful Kharne the Betrayer is an avid follower of Khorne and an indispensable addition to any Khorne army.
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Tzeentch:
Tzeentch is the god of change, and embraces sorcery and fire. A Tzeentch Commander has the choice to take Chaos Space Marine Sorcerers, who are essentially psykers. They may also take pink/blue horrors and flamers of Tzeentch, as well as the Lord of Change Greater Daemon. The powerful sorcerer Ahriman Of The Thousand Sons can also be used. Each unit is fine-tuned to take advantage of powerful sorcery and intelligence, but failing in sheer strength.
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Nurgle:
Nurgle embodies pestilence and disease in every form. The plague marines that follow him are much tougher than other marines, able to take more damage rather than give it out. Plaguebearers and Nurglings, who are essentially cannon fodder, both embrace this unique ability to take more damage than most other standard units. The great unclean one in this god's greater daemon, and can survive quite a long time.
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Slaanesh:
The god of pleasure and the corruptness that comes with it, Slaanesh manifested thanks involuntarily to the Eldar. Noise Marines, who create unthinkable noise, follow Slaanesh. Other followers include the Daemonettes (and the mounted variant), as well as the fiends of Slaanesh. Not forgetting of course the Greater Daemon prince of Slaanesh.
While there are other lesser gods mentioned, these are the ones that affect a Commander's army in terms of units.