"Therapists" can be social workers or psychologists or psychological associates. The types of psychologists who do therapy have Ph.D.'s, Psy.D.'s, or perhaps M.A.'s. (Although in the U.S. people have to have a doctorate to be called a "psychologist.")

I don't think there is much difference in the quality of the therapy they do. I prefer social workers because of my bias that they tend to be more down to earth.