Psychology is the study of behavior and the things that brains do (whether it's abnormal behavior or not is irrelevant). Psychiatry is the administration of remedy to aberrent people. Huge difference.the field of psychology is exactly as you said.... the study of mental processes. But the difference between a psychiatrist and a clinical psychologist is exactly what the other person said.... one is a medical doctor capable of dispensing meds, the other is a doctor in the sense that they have a pHd, but can not dispense meds. The medical background, and ability to dispense medications, and perhaps a slightly different tactic in which the two might approach a problem, is really the only difference.
While not all psychologists are clinical psychologists, many will take that path, and see just as much abnormal psychology as a psychiatrist.
At any rate, all people that go in for the study of either are not "whacked" as Curio said, any more than all people that drive are extremely lazy.