Straight Edge (sXe) is a more philosophical offshoot of the punk movement, a reaction to the hedonism and self-destruction that characterised punk. The basic tenet of the philosophy centres around the issue of self-control. The goal is to regain as much personal control over your own life as possible. sXe is the only youth counter-culture to actively discourage drug use, alcohol use, and casual sex.
Straight edge is a lifestyle centred around personal development and well being, while encouraging fun and togetherness. sXe is not just about being into contemporary punk music acts and being against drugs.
It goes deeper than this. The movement wishes to attract people away from dependancy lifestyles centred around drug habits (legal or illegal) and unhealthy and exploitative eating and general living habits common in modern cultures. sXe is not dogmatic, there are no hard rules, these are for you to decide. Nobody should dictate like
the police, or preach an ideology like the church or state.
The term itself is believed to have been coined by the band Minor Threat of the punk rock/hardcore scene in the early '80s. Their singer Ian MacKaye eschewed the nihilistic tendencies of punk rock, promoting instead the simple (almost simplistic) philosophy of "don't drink/
don't smoke/don't fuck."
The original definition of straight-edge only included the rejection of mind altering substances and promiscuous sex. It has since been expanded to include a vegetarian or vegan diet, abstinence until true love and commitment, an anti-abortion stance, and sometimes compulsory heterosexuality. All of these are dependent on the individual and the particular scene - some scenes are very militant about some of these, particularly veganism, while others are relatively open.
The X on the hand comes from the practice of marking minors' hands at all-ages punk shows so they couldn't buy alcohol.
Information taken from the sXe FAQ.