Psychiatry is the specialist medical discipline dealing with the diagnosis, investigation and management of mental illness.
Diagnosis primarily involves a detailed assessment of symptoms (by asking questions based on criteria defined in diagnostic manuals), the use of screening/diagnostic questionnaires and where necessary the use of special investigations.
Special Investigations include medical (blood tests, brain scans etc) and psychological (e.g. personality inventory) tests.
Treatments include evidence-based biological (e.g. medication), and psychological (e.g. Cognitive Behavioural Therapy) approaches.