Cutting and other self-harming behaviors are coping mechanisms in response to stress or anxiety, depression, compulsive feelings, feelings of emptiness, or other distressing emotions. The act of cutting releases endorphins, producing calm relief or even euphoria. While not chemically addictive, self-injury can be extremely difficult to stop simply because it is so effective, despite its attendant risks and overwhelming social stigma.
Self-harming behaviors are, by definition, NOT suicide attempts, although cutting can result in serious infections or accidental death - or more often, trips to the emergency room for stitches.
A popular assumption is that most cutters are young women, often with a history of psychological trauma, abuse, or loss. The demographics of cutters are actually difficult to determine - in part because, contrary to another common belief, most cutters go to great lengths to conceal their behavior.
Female cutters often self-injure in less publicly visible areas, such as their thighs, and/or wear jewelry, makeup, and clothing meant to cover up their wounds. Some cutters who have many obvious scars nonetheless are ashamed of their behavior and won't admit to it if questioned. Cat scratches or shaving cuts are common excuses.
A wider definition of self-injury includes, in addition to self-burning, behaviors such as banging one's head, hitting oneself, and scratching or pinching oneself, which are more common than cutting among young people of both genders.
The demographics of cutting may be changing as the behavior is more widely publicized. People who formerly practiced some of these other less harmful behaviors may be tempted to experiment with cutting. If they continue to do so, it is probably because they find cutting to be satisfying or effective, NOT because the behavior is or ever has been "trendy."
As for the association with emo music and culture, most (but not all) cutters are young people, and many young people listen to emo bands. In addition, people who are distressed are often attracted to music that relates to what they are going through. However, since most cutters conceal their behavior, it may be only a small minority that vocally identifies with emo culture.
The maliciously joking attitude that many people have toward cutting may be due to the disgust, confusion, and horror that the idea generally provokes. Becoming educated about cutting can diminish those feelings and help to create a supportive attitude toward friends and family members who struggle with self-injury.
*This is not mine.*