Psychiatrist:

Medical doctor who is specialized in the diagnosis and treatment (including medication, psychotherapy and psycho-social treatment options) of emotional disorders. These physicians usally cooperate with other mental health professionals of mental, addictive and emotional disorders like depression, anxiety disorders, obsessive-compulsive disorder, substance abuse or psychoses (e.g. schizophrenia). In some countries, most psychiatrists are also psychotherapists, in other countries not.

Psychotherapist:

Specialist in giving psychotherapy. Some psychotherapists also are medical doctors, some are not. Some psychotherapists specialize in only a particular psychotherapic method. In some countries, psychotherapists are usually also psychiatrists, in other countries not.

Psychologist:

Specialist in psychology. Some psychologists also have psychotherapist competence, but not all. Psychologists make investigations, diagnosis and prepare reports. Many psychologists are also psychotherapists.

Social worker:

Specialist in giving social aid and investigating people with social problems. Often employed by government social aid organisations.

Physician, medical doctor

Specialist in the diagnosis and treatment of one or more diseases.