PSYCHIATRIST These are medically trained Doctors who have chosen to specialise in treating mental disorders. The only ones qualified to prescribe medication. Some psychiatrists have undergone extra training in therapy and offer similar things to psychologists or psychotherapists.
PSYCHOTHERAPIST Psychotherpists are usually highly trained in one particular type of therapy. They have an in depth knowledge of that therapy (eg Psychodynamic), but are often not overly familiar with other forms.
PSYCHOLOGIST Anyone who has obtained a Psychology degree is entitled to call themselves a Psychologist. However, there are many different kinds of Psychologist, and the people who are qualified to offer therapy are usually called Clinical Psychologists or Counselling Psychologists. These people undergo an extra 3 year training which usually qualifies them up to a Doctorate level. This can be a little confusing as they are Doctors of Psychology, not medicine. They are usually trained in a variety of different therapies and are in a good position to assess you for the type of therapy you might need, and often are able to provide it.