Psychiatry is the branch of the medical field which specializes in diagnosing and treating mental disorders. Psychiatrists are doctoral level physicians who have completed medical school, plus a multi-year residency in psychiatry in a hospital setting. As medical doctors, psychiatrists are able to prescribe medicine, and to engage in medical and physical treatments for mental disorders.
Thirty to twenty years ago, it was pretty common for a psychiatrist to practice psychotherapy, but these days it is less common. As a field, Psychiatrists have gravitated away from therapy and towards the usage of medicine as treatments of choice for treating mental illness.
Having a Psychiatrist involved in treatment planning is essential for moderate to severe forms of mental illness where the state of the art treatment protocols necessitate the use of medicines. Bipolar illness and schizophrenia are good examples. Some (not all) psychiatrists also are expert at administering electro-convulsive shock therapy (ECT), a remarkably effective treatment of last resort for medication and therapy-resistant forms of depression and schizophrenic catatonia.