A selfish and self-destructive way to garner attention or release stress. Often turned to by people unwilling to make or incapable of normal human relationships. Most cutters will tell you that it helps with their problems, but it's mostly psychological. They expect it to help so, in their mind, it does. In my observations, however, it has only lead to increased problems down the road.
Cutting is a way to "deal" with your problems without actually solving them.