It's a term describing the foolishness of poseurs. Poseurs WANT to be labeled (Punk , Goth and several other subcultures are about not labeling), so they come up with specific dress codes for a group.
Punk:
Plaid Pants (Think Avril Lavigne), Tie (again, Avril), Combat Boots, Jeans (although this one is rather accurate), spiked bracelets, chucks and bondage pants. Think MTV's Power-Pop bands, like Sum41, Blink182, and Good Charlotte.Recently it has become important to know how to skate as well...
Emo:
Tight clothes (you are skinny because you are a vegan), thick, horn-rimmed glasses, greasy hair, messenger bag...just look dorky. Be in a lousy whiney band, complain a lot.
I could name more stereotypes like this, just check out sykospark.net