*music23*

"Therapist" is a very general term for all of those types of people.
ONLY psychiatrists are MDs. In general, they are the only ones that are able to prescribe medications, though in a few states I think it may be different. Psychologists are sometimes called "doctor" as well, but they are not MDs. They often have PhDs.
Social workers and counsellors are also considered therapists, but their qualifications aren't as much as the above. Regardless, they can still be vital to a person's therapy.
All of these people are licensed to do "talk" therapy, but if you see any psychotherapist besides a psychiatrist, they (usually) can't give you medicine, and if they believe that your problem(s) are beyond their expertise, they should refer you to someone who can better take care of you.
I hope that helps.
Kristina