Self injury is the deliberate injury to oneself through cutting, burning, hair-pulling, head banging, bone-breaking or self poisoning.
Injuring yourself on purpose by making scratches or cuts on your body with a sharp object enough to break the skin and make it bleed is called cutting. People may cut themselves on their wrists, arms, legs, or bellies. Some people self-injure by burning their skin with the end of a cigarette or lighted match.
When cuts or burns heal, they often leave scars or marks. People who injure themselves usually hide the cuts and marks and sometimes no one else knows.
The dangers of cutting are obvious, such as the risk of losing too much blood or of getting an infection. But there are hidden emotional dangers in cutting too. It can become enough of a habit that some people feel that they are addicted cutting/self injury. 
Reasons Why Adolescents May Self-Injure: 
What Can You Do To Help A Friend Who Self Injures? 
 El Paso County Department of Health and Environment, Safe Teen Self Injury Fact Sheet
 The Children’s Hospital website, September 2007