According to the American Psychological Association, a psychologist is a trained professional with a doctoral degree who practices evidence-based treatments to help with various mental disorders. Psychologists work in private practice or in groups with other medical professionals.
Wikipedia explains that psychologists are professionals who evaluate, diagnose, treat and study behavior and mental processes. The APA notes that the major difference between psychologists and medical doctors, including psychiatrists, is that psychologists are generally not allowed to prescribe medication. Instead, psychologists prescribe counseling and other behavioral treatments and work in conjunction with physicians to prescribe medication for their patients if necessary.