Someone who tries very hard to be something they're not but never succeeds.
A poser trying to be a "goth" will wear jeans and a band t-shirt. The band being a band they've only heard one song by.
I used to be a poser, then I saw sense. I'm just myself now. My own Style. Simply just, me.
Sindy - "OMGZZZZ CHECK MY SLIPKNOT T-SHIRT OUT!! I LOVE IT! N SLIPKNOT, THEY'RE THE BEST BAND EVER!"
Lucy - "Umm... what songs you heard then?"
Sindy - "Can't remember the names! *awkward laugh*"
Lucy - "Yeah..."
But posers are often mistaked as actual goths by chavs (townies, scallies). But chavs are stupid and blind.