Most people you will meet in life; People who do things because it will improve their social standing, not because they actually enjoy/care about what they are doing are considered posers. Should be seen as a subculture, albeit a parasitic one that hijacks any other given subculture, waters it down, makes it "trendy" and causes people who still try to stick with the given subculture to look like a walking cliche. A good analogy is when a corporation donates to a large charity, then puts up commercials about it, proving (9/10 times) that they donated to appeal to the masses, not because they want to make the world a better place. Also, for racial minorities, an equivalent of poser would be a "ghetto rat," a "gangster," or any other stereotype.
tl;dr: Poser is a term used to describe those who cause culture and subculture to become shit.
The way 'hipster' has become because of posers, compared to the way it was before most people even knew about it. The way 'emo' (or Post Punk) has become, largely because of posers that were previously identified as 'scenesters.' The way 'Goth' and 'Punk' have become, due to kids seeing Marilyn Manson and thinking that dressing a certain way would help them rebel against society.
By SadDino