A poser is essentially everybody in High School who actually pays attention to fashion/studies how to conform to a specific group. Generally, someone who is socially awkward and/or physically unattractive gets called a poser when said individual tries to fit into a certain group, usually comprised of people who have been studying how to conform for a longer period of time. However, since all High School cliques are based on extreme superficiality, the word poser could apply to everyone in such a group. The idea that anyone is, in fact, a "real" punk/goth/hipster, etc, usually stems from how much dedication they have to putting together their personality. It's usually born from a psychological condition that has been impeded into their brain through corporate propaganda; more or less, the idea that you need to consume/conform on some level to have an identity at all.
When you are using external sources and symbols to define your personality, you are not actually being "real" in any sense, and that's especially true if the symbols have been given their meaning independently of yourself or peers. Peer pressure and conformity, of course, are normal adolescent behaviors, although teenagers take this to an extreme in consumerist societies (and corporations exploit this condition in teenagers as well, hence how important the word "poser" seems to be here on urbandictionary).
It should be noted, however, that some hipsters take this thought process deep into their twenties, and never fully mature past the point of making an identity out of secondhand symbols or processes of thought (although this behavior has more complexity and nuance to it, and has more to do with boasting of education obtained at a university level, and classism). There are an enormous amount of college graduates in western society with uniform tatse/clothing styles/mannerisms, with not only identical opinions, but identical expression of those opinions, and identical attitudes.
So, basically, "poser" is an Orwellian word, that instills fear into the minds of teenagers (and some adults), which in turn makes them consume/conform in an efficient manner to avoid being labeled as such. For corporations, this is one of many tactics that help to create a very effective society, where individuals place their own constraints instead of having those constraints come from a dictatorial system.
Kid A: Man, that kid is a poser! He doesn't even know any other Greenday albums besides "American Idiot"
Kid B: Well, you shouldn't be too proud of knowing Greenday albums in the first place, because they aren't a very talented band. I would say they are an embarrassment to punk, but punk was never anything special in the first place. It was all Malcolm Mclaren's ploy to sell clothing in London in the late 70's.
Kid A: Dude, you're faggot!