hillbunnyb pretty much nailed the difference. Psychiatrists are medical doctors whereas psychologists are not. Psychologists generally practice psychotherapy ("talk therapy") while Psychiatrists generally practice psychopharmocology ("prescribing medications") -- though there are psychiatrists who do both.

Incidentally, clinical social workers, licensed professional counselors, and marriage and family therapists are all similar to psychologists in that sense...they are trained to diagnose and treat mental disorders through the use of psychotherapy. (If they think the client needs meds, they'll usually refer them to a psychiatrist.)