Doctor of psychology. Psychotherapy is defined as a therapeutic interaction or treatment contracted between a trained professional and a patient; family, couple or group. It may be performed by psychologists, psychiatrists, counselors, school counselors, art therapists, etc. Clinical psychologists are doctors that specialize in diagnosing psychological dysfunction; psychological testing; and psychotherapy services.
Therapy vs meds. Speaking very generally, most psychologists hold either a PhD or a PsyD degree (I hold both) and practice psychotherapy. Psychiatrists hold either an MD or a DO degree, and they prescribe psychotropic medications, and generally do not do all that much therapy these days, but years ago psychiatrists would do both. Social workers, psychiatrists, and psychologists all do counseling/psychotherapy.
Can be lots . States regulate these definitions, most psychologists are trained at a doctoral level are specifically licensed as psychologists and complete a one year apa approved internship and a year post-doc residency, then must pass state and national exams. See: http://cpancf.Com/articles_files/psychologicalcopingcancer.Asp.