This is an important question because the distinction reveals the boundaries of what each can do–and overlap.  FPAMed, a long-established Forensic Mental Health Group of providers produced a video that describes the training and skill of both in such a straightforward manner that I recommend you take a few minutes and get the skinny from a practice of both psychologists and psychiatrists.  The video is long but answers are at the beginning and the full video is worth a watch, in my opinion.