From the American Association of Director of Psychiatric Residency Training website: A psychiatrist is a physician who specializes in the prevention, diagnosis, and treatment of mental, addictive, and emotional disorders. He or she understands the biological, psychological, and sociocultural components of mental illness and is qualified to order diagnostic laboratory tests, to prescribe medications, to evaluate and treat psychological and interpersonal problems, and to give continuing care for psychiatric problems. The psychiatrist is also prepared to help individuals and families who are coping with stress, crises, and other life problems.

Psychologist are social scientists and researchers, but they are not medical doctors.

From Psychology Information Online website:

Psychology has been around as an academic and research science for over 100 years. Psychology is a social science, and psychologists have contributed a large body of research to our knowledge about human behavior, human development, psychological problems, the measurement and understanding of personality characteristics, and other important areas of knowledge about how people think, feel and behave.