A poser is someone who tries to fit into a profile they aren't. People who try to give off the impression that they are one thing when they are really another.
Also, a poser can be one who says they can do something that they can't.
Or more commonly, a poser is someone who, when doing something successfully, takes more than enough credit for it.
* These videos are coming directly from Youtube, they may or may not be most relevant to the word "Posers"