A hipster is someone who does not identify with any sub-culture because they consider themselves independent and of a counter-culture. They often identify as a counter-culture leading to anything that is popular being labelled as "too mainstream" by hipsters (however this in fact is often more used by their critics to parody them for example "being a hipster is too mainstream for a hipster" however in fact this may be accurate considering that most hipsters will not self-identify as hipster). They tend to express progressive attitudes but because it is often mixed in with a militantly elitist attitude they are generally disliked by non-hipsters. They have a fashion sense that often includes things like ripped jeans and American Apparel products. They tend to identify with indy music although occasionally they also identify with classical (which is just more ammo for their critics who believe it is part of their faux intellectual and elitist persona).
Bob: You are such a hipster!
John: No I'm not!