Self-harm is an umbrella term for any behaviour which can cause damage to your health. This can include cutting, but also includes overeating, taking drugs, smoking and drinking too much alcohol. However, the last two are seen as socially acceptable, whereas cutting is not. Examples of self harm might be: cutting, burning, pulling hair, picking skin and deliberate bruising. Self-harm is usually conducted at times of anger, distress, fear, emotional worry, depression or low self-esteem, and is done in order to manage negative feelings that the self-harmer cannot deal with in a rational or logical way.
Self harm is not a suicide attempt. In fact it could be described as exactly the opposite - someone in great distress may self harm so that they don't kill themselves. It is a survival and coping strategy.
Who self harms and why? I'm going to talk specifically about cutting here. Anyone in distress could self harm - there are very few stats on numbers and distribution. 20 years ago, it was estimated that 300,000 women presented at A&E having committed an act of self harm. If one realises that most self harmers do it in secret and never seek help, then that figure rises alarmingly. It is particularly common in teenagers ( sometimes as a copying thing) but people of any age do it, although it tends to burn itself out as people get older.
People who self harm describe the absolute compulsion to do it. When that compulsion is present, it is almost impossible to resist it. No one knows exactly what happens physiologically, but the act of cutting oneself provides relief from intense emotional pain. That's why it's used as a coping strategy. People say they do it to feel real because of the pain; some do it to see blood to prove they are still alive. It's believed that anger is involved - it might be too frightening to get angry with the person who has hurt you, so the anger is directed inwards and got rid of by cutting.
But no one really knows.
I spent 16 years in the RAF defending the Free World , then got bunged out unceremoniously for being bipolar. I and was subsequently diagnosed with PTSD. Funny old world, isn't it?