Theoretically, a poseur is a person who acts like they are an expert in a given field, while nothing could be further from the truth.
In reality, however, the word "poseur" means "everyone except me". Yes, whatever you're into, from the Beatles to medieval antique butt plugs, you can always be pretty certain that you know a lot more about it than anyone. Everyone else who claims to be into it is just posturing, and their meager experience with it is dwarfed by your own God-like wisdom. After all, you liked butt-plugs before they were cool, and that's all that matters. A poseur can also refer to someone who attempts at using a stereotype to create a self-image that truthfully does not portray who they really are.
A poseur may also be infected with a particularly nasty virus.
Faggots spell it poser, under the false impression that misspellings makes them trendy, which supposedly makes them more edgy in turn. Ironically, most people who spell it this way tend to be poseurs themselves.
Either way works, it all depends on how gay you are.