A psychiatrist is a medical doctor who has decided to specialize in mental health. Most people see a psychiatrist for mental health medication. Some psychiatrists also provide psychotherapy, in addition to medication management. They are the only psychotherapy providers who can prescribe medications.
A psychologist is an individual with a doctoral level degree in psychology. People typically see a psychologist for psychological testing services and psychotherapy.
Counselors and therapists are similar, and the terms are often used interchangeably, although there are some minor differences. Counseling is often thought of as a shorter duration of treatment or a brief intervention, whereas therapy is sometimes referred to as more long term treatment.