It's a roleplaying thingie. Not characters, us, but sorted into Houses to recreate the Hogwarts experience. You fill out a questionaire, get sorted by the majority, and then can hang out in your house's common room. You earn points for your house by doing homework for the classes (making icons, answering questions, possibly writing fic, I'm not sure), by entering contests, by playing "Quidditch" via debate, etc.

hogwarts_ch is just the sorting community. There's hogwartsch, which is the main one, plus "Diagon Alley", an icon community, and each of the house common rooms.

Wanna play? You have to have a referral now, 'cause they got swamped, but I'd be happy to. :)