Cutting, or self-injury behavior, is a very complex behavior. This information page explains what self-injury behavior is as well as appropriate ways to help those who self-injure.
Below are some other resources for self-injury.
To Write Love On Her Arms
http://twloha.com/
SiOS - Self-Injury Outreach and Support
http://www.sioutreach.org/
S.A.F.E. Alternatives
Self Abuse Finally Ends
http://www.selfinjury.com/