According to the 4chan masses, hipster is anyone who likes something you don't. Because everything you don't like is poop, and people who would enjoy said poop are doing it ironically because they think its too DEEP for you.
At least, that's what I've managed to piece together. The word does get thrown around pretty liberally, I've seen it used against Team Ico fans, the Scott Pilgrim fanbase, Valva fanboys, Mac users, people with Facebook, fans of Big Bang theory, and even people who love retro games.