Psychiatrists are medical doctors and psychologists are not. The difference in the words' meanings is reflected in their roots; the suffix "-iatry" means "medical treatment," whereas "-ology" means "science" or "theory." Psychiatrists can prescribe medication, whereas psychologists can not.
Psychiatry is devoted to the study, diagnosis, treatment and prevention of mental disorders. To qualify as a psychiatrist, one must earn a degree from medical school and complete four years of residency in a mental health setting such as a hospital's psychiatry department. The license to practice psychiatry is granted after the completion of the residency.
On the other hand, psychology is the study of mental functions and behaviors. Psychology aims for a better understanding of people by establishing general behavioral principles. Psychologists typically attend five to seven years of college to obtain a doctorate degree. Those more interested in clinical psychology may choose to earn a PsyD instead of a PhD. Typically, a one- to two-year internship is required before a license is awarded to practice psychology.
Psychologists are more likely than psychiatrists to perform tests on a patient's personality. These tests range from questionnaires to Rorschach tests and even neuropsychological tests, which evaluate brain functions.
Both psychiatrists and psychologists treat a wide variety of disorders, and both can do research.