I got no urge to binge and purge!"
Straight Edge is a philosophy in which participants abstain from the vices of the modern world. What these actual vices are is subjective, but most commonly, followers will keep well away from alcohol, nicotine, THC, other drugs, sometimes premarital or promiscuous sex and in some cases even caffeine or meat-based diets.
Thought to have stemmed from the beliefs of Minor Threat front man Ian MacKaye, nowadays Straight Edge is held all over the world throughout numerous different subcultures. It's also believed that The Modern Lovers helped plant the seeds of Straight Edge with their song "I'm Straight".
It has at times been accused of initiating violence, particularly in the Hardcore Punk scene, sometimes causing it to be labeled a gang. Nowadays, however, Straight Edge is largely peaceful.
While the Straight Edge punk subculture began in the 1980s, the violent version most famous in media emerged in the mid-1990s with the pioneering metalcore band Earth Crisis, who wrote songs with a stronger Drugs Are Bad message, including a song that encouraged listeners to "kill your local drug dealer". When the mainstream media heard about this, it sparked a minor moral panic and led to claims that Straight Edge was a gang, in reality never reaching the level of organized violence claimed by the media.
The Straight Edge subculture continued to be influential in metalcore and hardcore punk at the Turn of the Millennium, and a number of prominent hardcore bands emerged from the subculture at the time. People in punk continue to identify with the culture to this day, though it's less prominent than it once was.
It is notable that MacKaye himself has claimed many times to have never intended to start "Straight Edge" as a social movement and, despite continuing to live a clean lifestyle to this day, he does not consider himself a part of it.