"All this excitement and torment is getting me SO DAMN HORNY! Anyone of you, please ram this chainaxe deep inside me and rev it as hard as you can!"
Unlike the other three Chaos Gods, Slaanesh was born many millenia after Chaos' creation. She was the product of the Eldar Empire's descent into hedonism and extreme debauchery; their collective emotions coalesced in the Warp into a corrupted entity that would crave only the most extreme emotions. As She had been unwittingly created by the Eldar, Slaanesh claimed the souls of the Eldar and their Gods as its own, consuming all of the gods save for Cegorach, Isha, and Khaine. Khorne directly confronted Slaanesh to claim Khaine as his own, while Nurgle claimed Isha for himself. Cegorach managed to escape into the Webway, vowing to undo Slaanesh's evil and save the Eldar. All future Eldar now had to guard their souls from Slaanesh, whom they call "She who Thirsts."
Slaanesh's champions are maniacs who get off on pleasure and pain. They simultaneously embrace the seductive and horrific, and will frequently torment their victims ceaselessly. Much like the prince, hermaphoditism is common among among both daemons and mortal followers.
Slaanesh hasn't done very much so far, other than steal Khorne's chainaxe to use as a sex toy, and to creepily hit on Isha in the warpchat. Said warpchat is also used to host his/her/their/its camshows - or else scam people out of their credit card info. He/She/They/It also flirts with Khorne, turning his own threats against him as he/she/they/it gets off on being demeaned. Magnus claims that Slaanesh has a "weird sense of humor" with regards to gelding mortals.
She/he/whatever almost invaded Commoragh, being summoned by Leman Russ during the "Mon'keigh baby game" session with his Dark Eldar "captors" (turns out, that they were actually stuck with him, not vice versa). One of them was devoured by Slaanesh when he tried to pronounce her true name. As she was preparing to unleash his horror on Drukhari, Russ fell asleep and the influence ended, to her disappointment (this pleased her). The experience left sole surviving Drukhari guard in a crying mess before being dragged off by the Cabal to fight in the arena.