1. A poser is most commonly defined as someone who acts in a manner inconsistent with their personality. One who tries to be something they are not.
2. One who accuses others of being poseurs, or more commonly "posers," in order to up their own social status, so that they themsleves will not be labeled as a poseur.
I was walking through Borders the other day and I heard this Poseur talking about how he/she loves emo and listens to Miss. A. Lavigne all the time.