Psychopathology is the scientific study of psychological disorders. If you have a physical illness, you go to your GP and they will diagnose your illness. Psychological disorders are a lot more complicated.
Abnormality
a psychological or behavioural state leading to impairment of interpersonal functioning and/or distress to others.
CLASSIFYING PSYCHOPATHOLOGY
There are two classification systems that are used to diagnose psychological disorders.
The International Classification System for Disease (ICD-10), from the World Health Organisation.
Diagnostic and Statistical Manual for Mental Disorders (DSM-V), from the American Psychiatric Association. It uses the Global Assessment of Functioning (GAF).