A poser is any individual who:
a) attempts to be something he or she could never be
b) imitates fiction to the point of vain idolatry
c) seems "pretty fly for a white guy."
Under this broad definition, the vast majority of people are posers, or wannabes, or Holden Caulfield's "phonies." See the Project Mayhem, Vanilla Ice, or King Missile for issues or examples related to poserdom.