They both treat abnormal behavior, dealing with the "mind". In the States, a psychiatrist also holds a degree in medicine and is able to prescribe medication, as he is also dealing with the brain itself. Simply put, a psychologist works with feelings, actions and emotions; a psychiatrist works with physical, neurological or chemical abnormalities that cause certain actions.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.