A person who says that they love a certain style of music or clothing, (etc.) but really doesn't, and is trying to fit in.
Emily: Oh my God that girl is a poser! She doesn't like Green Day! I am the biggest Green Day fan!!!
Angela: Emily there are a ton of people who love Green Day much more than you. You are not their #1 fan.
Emily: Well I consider myself their #1 fan.
Angela: You only have one CD, there is a ton of Green Day CDs.
If you only have one CD of a band because you think it will make you cool, you are identified as a poser.