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.