What is the difference between a psychotherapist, a counsellor, a psychiatrist, and a psychologist?

Psychotherapist

Psychotherapists are usually highly trained in one particular type of therapy. They have an in depth knowledge of that therapy (e.g. psychodynamic), but are often not overly familiar with other forms.
UK Professional Body: United Kingdom Council for Psychotherapy

Counsellor

These people are usually trained within one particular form of therapy. That is usually person-centred counselling, but not always. They will have been trained in counselling skills and will possess a counselling qualification.
UK Professional Body: British Association for Counselling and Psychotherapy

Psychiatrist

These are medically trained Doctors who have chosen to specialise in treating mental disorders. They are the only ones qualified to prescribe medication. Some psychiatrists have undergone extra training in therapy and offer similar services to psychologists or psychotherapists.
UK Professional Body: Royal College of Psychiatrists