A psychiatrist, unlike a psychologist, is a medical doctor with an M.D. degree or D.O. degree. Like all medical doctors, psychiatrists complete an extensive residency training program after graduating from medical school. Therefore, psychiatrists are able to prescribe medications, unlike psychologists.
Psychologists typically hold a PhD in psychology, as opposed to a medical degree, because they do not attend medical school. Therefore they are able to provide counseling and therapy to treat mental health, but would have to refer patients to a psychiatrist or other M.D. to provide medication for a patient.