Generally coming from Hollywood slang for a better, more fun party, occurring after another, more boring, party.
May also be used to describe activity that occurs after the club/bar scene is closed for the night.
"Seriously hate these work-related shin-digs."
"Don't worry. There's a kick-ass after party at Kate's. We just have to lose the big-wigs."
"Why do the clubs close at two?"
"S'all right- after party at Tom's- just stick w/ me."