What constitutes hipsterdom, anyhow?

I dunno if anyone cares to enlighten me, but there's a certain term of abuse in heavy rotation these days - hipster - the exact meaning of which somewhat eludes me. The people who use it are invariably "cool" people - people who are, in fact, according to past non-pejorative uses of the term, "hip;" they seem to mean the term to refer to people who are "trying" to be cool, but really aren't. Indubitably there are people like this out there, but I'm never exactly sure who they are, so I can't really comfortably use the term. Do people who use the term actually have a sense unknown to me - akin to gaydar, perhaps- where they can scan the crowd at a gig and pick out the hipsters versus the hip? How do they differentiate? Are they ever fooled? Most troubling: does one need to partake of hipster-like values in order to recognize them in others? Could "hipster" mean, really, people like us?

Or, well... I guess it can't mean me, since I have no idea what the term means...