While both psychiatrists and psychologists are mental health professionals, the big difference is that psychiatrists are medical physicians (DOs or MDs) while psychologists are not. Because they are licensed physicians, psychiatrists can prescribe drugs. Psychologists are not allowed to do that.
edit Education and Topics of Study
All psychiatrists complete at least four years of post graduate medical training.
Psychologists are not medical doctors; they do not have medical degrees but have advanced degrees in clinical or counseling (like Master's in Psychology) or the equivalent and have more training in psychological research and personality assessment than DOs or MDs.
There are some natural overlaps between a psychologist’s work and a psychiatrist’s work. Psychologists study the mind that includes the study of disturbed mind, and even a sick mind or psychopathology. Psychiatrists also study psychopathology.
edit Nature of treatment
Psychologists and psychiatrists often work together for the well-being of the client as their job descriptions overlap. However, there are differences in the techniques they use. Psychologists use approaches such as counseling, hypnosis, psychotherapy and relaxation. While psychiatrists can use these techniques too, they are also allowed to prescribe medication to their patients. For diseases like schizophrenia, for example, the patient would see a psychiatrist and not a psychologist.
The fields of psychology and psychiatry are both essential in researching and developing treatment for improving mental and emotional health. Differences aside, psychologists and psychiatrists share a common goal and that is helping people feel better.
edit Video explaining the differences