Psychologist vs. Psychiatrist vs. Psychotherapist

A psychologist is an expert who has a Ph.D. in psychology. They usually work in one sphere of psychology (e.g. child psychology) and typically deal with healthy people.
A psychiatrist has a medical degree and specializes in mental disorders and their treatment.

A psychotherapist may be either a psychiatrist or a psychologist depending on their academic degree. They help people overcome light or moderate problems such as psychosomatic disorders, depression, eating disorders, and phobias.