There is a virtual alphabet soup in the letters behind the names of mental health professionals. Here is a short form answer to explain the differences.
Psychiatrist, M.D. is one who has gone through medical school, just like an internist or a surgeon. Their area of focus however, is studying how the brain works. Some psychiatrists also psychotherapy (counseling), but most do not. Their predominant role is to prescribe and monitor medication. A psychiatrist is also the practitioner who would administer ECT.
Psychologist- letters are either Ph.D. or Psy.D. This is a person who has gone through a doctoral program of psychology. They have also completed an internship approved by the American Psychological Association and passed an exam. Psychologists do not, (except in a handful of states) prescribe medication.
Professional Counselor- LPC. This person’s master’s degree or Doctorate, must have been in a program dedicated to Counseling. To be licensed, one must only achieve a Master’s degree with a doctorate being optional. These individuals have also completed internships and passed an exam at either the state or national level. Counselors do not prescribe medication.
Clinical Social Workers- To be identified as a social worker, the individual’s masters or doctoral program must have been in social work. There are social workers that deal primarily with social systems, and/or grant writing. A clinical social worker, is more often than not, working in a mental health facility or private practice with patients. Like counselors, a master’s is required for licensure, and a doctorate is optional. Social workers also have internships and exams. Social Workers do not prescribe medication.
Therapist- or psychotherapist. While the names above are protected by licensure, the names of therapist or psychotherapist are freely used by any of the above individuals. This is because we are all practicing the skill set of psychotherapy, which is the educated art of developing a relationship with a client or patient, for the purpose of healing a mental condition.
I use the term largely because I have a Master’s in counseling, and a doctorate in social work and am thus legally permitted to identify with both organizations. Therapist is a more simple way of describing what I do.