The Diagnostic and Statistical Manual of Mental Disorders (DSM) is like a big book that lists out different mental illnesses, and explains what those illnesses are. It's like a big reference guide for mental health professionals so that when someone goes to them for help, the doctor or therapist can look up their symptoms in the DSM to see which mental illness might be the cause. The DSM helps doctors and therapists figure out which treatment plan might be the best for a person.