The DSM-IV (Diagnostic and Statistical Manual for Mental Disorders, 4th edition) is what mental health professionals use to formulate official diagnoses. Mental health problems are not as tangible or measurable as physical ailments, so the manual is used to assess tendencies, frequencies, and duration in order to discern likelihood of the presence of a mental illness. It is not cut and dry, but there are very specific indicators for each disorder. If interested, one can learn all about how the DSM-IV is laid out and referenced from The Virtual Psychology Classroom.
It is important to understand that disorders are divided into classes:
- Anxiety disorders such as post-traumatic stress disorder (PTSD), obsessive-compulsive disorder (OCD), and phobias
- Mood disorders such as depression and bipolar
- Psychotic disorders such as schizophrenia
- Eating disorders - anorexia and bulimia
- Personality disorders such as antisocial and paranoid
- Tic disorders such as Tourette's syndrome
- Dissociative disorders such as dissociative amnesia, dissociative fugue, depersonalization, and DID
I elaborate further about my experience with this disorder in a post called Growing up with Schizophrenic Mother.