Any one who does not follow his/her own ideas of what he/she likes. Following no one set norm makes you real and following no one set norm makes you poser. Poser is governed not by the clothes, music, or makeup you wear, but rather by the state of your mind and the conformity to your own beliefs.
A poser is someone who tries hard to be something they arent. Usually, posers call other people posers because they are jealous that the person they called a poser is more skater/stoner/goth/punk/rocker/grunge/etc. than they will ever be.