A Poser is a total showoff! The difference between a person with talent and a poser is that the poser is just trying hard to appear talented for personal status gain and to further boost his/her ego (which is already so overinflated that they unknowingly make fools of themselves with their over exaggerated shows) whereas the person with talent is just doing their stuff... performing (in the purest sense of the word). And would perform well with or without a crowd but the poser only performs when there is a crowd. So I totally agree with most definitions here up to a point... that being that, 1. yes, posers are trying to show off but 2. no, they are not trying to imitate some culture punk goth or otherwise but instead would just do anything to attract attention to the point of making a total fool out of themselves, unlike someone who just dresses punk, goth, rad or whatever (even if they aren't) but just felt like dressing that way to express themselves a little differently.
At the drumming contest that first guy really knew his stuff and played so well yet didn't make a big deal about it, but that second guy only barely passed as drummer yet he thought he was brilliant and didn't even realize he was making such a fool of himself... what a poser!