A set of rules that came from a Punk band called Minor Threat, now a trend for the teens of America to suck on. The rules are, no drugs, and no casual sex (only in a commited relationship).
Kids nowadays treat it like it's some fucking cult, or a group of people you have to be with to be cool (read; fashioncore
Kids who tell their friends they are Straight Edge, then turn around and go hang with their prep friends to smoke and drink to seem popular with all audiences are not rare and hard-to-come-by when it comes to Straight Edges.
They usually imprint X's on their arms for some reason or another with magic marker. More then likely, they feel the need to spread their houlier-then-thou shit eatting attitude. You might want to try attention whore
for some more info on that.
The teens of America have also turned and twisted the rules. Commited relationships to them, is basically a one night stand at their friends house with a girl they just met, where they bust out a condom they stole from their parents cabinet, without realizing the sole reason the condom was there was to prevent another fucking accident, such as themselves.
Oh, and drinking caffeine is ok, even though, sXe (as it's called) is another form of veganism, which is breaking those rules itself.
Hey, look at my arm. See those Xs I imprinted on my limb with a Super Sharpie marker? That means I'm Straight Edge LOLOLOLOL!