Loren will probably do a better job with this than I will (and I think we've already done this dance, oddly), but:

Back in the early-to-mid 80's, a group of hardcore bands, notably Minor Threat, started or were part of a backlash against the sex and drugs part of rock and roll. They nominally espoused the idea that drinking, doing drugs, and having casual sex was a bad idea. This caught on in a big way. The Straight Edge movement, named after a Minor Threat song, became quite popular, and the members identified themselves by magic-markering black `X's on the backs of their hands.

Straight Edgers are often noted for zero tolerance and disdain of all who don't follow their ideals. They're also known for often quickly and dramatically losing their ideals as they become legally able to drink.

Ian MacKaye, the singer and lyricist for Minor Threat, has, in more recent years, stated that he didn't intend to start a movement (with which he is usually credited) and personally hates Straight Edge. Whether this is true or a case of the aforementioned falling off the wagon I doubt we'll ever know.