Posers are extremely insecure people, usually teenagers and maybe a bit younger (11 or 12). Posers are extremely annoying, and pretend constantly, always putting on an act and pretending to be something they are not. In my opinion, a poser is like a tryhard, but is more concerned with being part of a scene (emo, goth, punk).
One of the most annoying things a poser does is pretend to be into a band. I had an msn conversation with a poser once.
it was like this:
i like Jimmy Eat World
yeah theyre great... whats your favourite song?
oh i cant really remember any songs off their album
you dont even know Jimmy Eat world you're a poser
that my friends, is a poser.
POSER: omfg my life sucks </33
POSER: just cause
POSER: i dont wanna talk about it
ME: you fucking poser emo