Chaos Gods

The four gods which rule over the Realm of Chaos in Warhammer Fantasy Battles and the Warp in Warhammer 40,000. They love nothing so much as dicking with each other, except perhaps dicking with their mortal followers. Each one is formed by the emotions of beings with souls. They are:

Khorne: The god of battle, honorable combat, and oh yeah, BLOOD! Formed by hate and rage, bravery and honor is also thrown in there.

Nurgle: The god of filth, pestilence, decay, and generally being a cool dude. Formed by despair and fear of death, acceptance and stoicism is also there.

Slaanesh: The god of pain, pleasure, and perfection. Formed by hedonism and excess, love and pussified creativity is also there. In 40k, the Eldar created him/her by having so many damn orgies they tore a hole in space. I shit thee not.

Tzeentch: The god of progress, knowledge, and Just as planned. Formed by hope, ambition, and the desire for change.

The Other Ones

Older editions of WFB make mention of several other lesser Chaos gods. These do not appear at all in 40k, and only sparingly in later editions of WFB, with some such as the Horned Rat not being generally acknowledged as part of the same pantheon. According to Lexicanum they are:

Be'Lakor: A fallen Chaos champion, who was apparently so crazy powerful that becoming a daemon prince was a demotion. Somehow still able to grant his followers unique magic powers.

Hashut: Father of darkness and god of the Chaos Dwarves. And if his followers are any indication, god of penis-compensating hats.

The Horned Rat: Patron of the Skaven, and god of...well, rats. Also disease, Chaos, etc., but mostly rats.

Malal: Renegade Chaos god. Interbutt lore states that he is the god of fractiousness and dissent, which given the company he keeps makes him a tad redundant.

Necoho: The god of atheism. I wish I was making that up.

Zuvassin: A minor god who just likes to see shit fall apart, specifically nonphysical stuff like schemes and lives. He and Malal are the co-chairs and so far only members of the Useless-Ass Gods Club (Necoho would join the club, but he doesn't believe it exists).