Punk is about thinking for yourself ^^ and not doing something you don't want to do to fit into society's 'standards'.
Of course you don't have to be 'punk' to think for yourself and stuff like that. But its just one of those 'groups' la.
The metalheads, the punks, the skins, the emos, the indie kids, scene kids, posers, cheenas, mat reps, jazzy people, rude boys, b-boys, rappers, pimps, skaters, bladers, bmx-ers, yo-yoers, wankers, gamers, ravers, nu ravers, pornstars, fishballers.. @_@ huh?
All are just a typical way to label someone, i'm sure not all punks like the same punk bands, and im sure not every metalhead enjoys listening to every genre of metal =/