Posers are people who "pose" as something they're not, pretend to look like a group of people, like punks, headbangers or stoners, usually related to music.
That was the simple way, now here's the real definition, according to me: Posers are usually people that don't fit in, so trying to fit in, they act like the people around them, pretending to like the music, have the attitude, and also dressing like them. I will use as an example the tipical poser stoner.
A lot of people don't like stoners, is it because they smoke weed, act like that or because they think that stoners are hippies or something like that, but i think stoners are cool people to be around, i would actually call myself one. But the poser stoner don't really think like his friends, so here are some of the things a poser stoner usually do:
smoke weed as a drug, not as a way to think about life and have fun
Struggle through Dark Side of The Moon and call himself a Pink Floyd fan, and when someone is talking about pink floyd, all he has to say is "they're great! i love their attitude! DSOTM is a great album!"
Can't chill and listen to a Jimi Hendrix song, he has to headbang or air guitar his way through the song
Fills his Ipod with Led Zeppelin, Pink Floyd, Jimi Hendrix songs, even though he hasn't listened to any of those since he was in his friend's.
Thinks surfing is a way to show off to his/her friends
If offered some drug as meth, coke or crack, he/she would deffinitely do it
That's all i know about posers, if you spot one, fuck him, let it be