A person, most commonly a girl in their teens, who has been through so much in their lives that crying (or sometimes they just can't cry)or any other form of emotional release becomes inefficient, therefore they resort to self-injury (aka self-mutilation, self-harm), but not intended to be a form of suicide. Self-injury is usually a sign of a serious mental disorder such as depression or bipolar, and caused from traumatic events such as death, sexual or verbal abuse, or low self-esteem.
Usually, proof of this behavior is well-hidden by wearing long sleeves and pants. Most self-injurers do not advertise their injuries. There is a such thing as 'attention cutters', or people who injure themselves just so others will feel sorry for them. Attention cutters usually purposely talk about their actions and make it seem almost like something they're proud of.
Cutters are often given the label of emo or goth, only because people are too ignorant to see that these people actually have pretty messed up lives. These stereotypes usually push these kids to want to hurt themselves more severely, thus those who have such narrow-minded views on cutters should shut their mouths and care about their own sorry lives.
"I hear she used to be a cutter, but since her family intervened, she's been to a counselor and is now on anti-depressants."
Stereotyped View: "Look at her over there, you can just tell she's a cutter because she has no friends and listens to emo music and wears black makeup."