A psychiatrist is a medical doctor that treats mental disorders with medication and/or therapy. Their training is usually the same as medical doctors.
A psychologist is someone who attends graduate school and is trained to treat, diagnose, research or educate others about mental disorders. Their training usually requires earning a doctorate degree and extensive hours of post-doc hours of specific training. There are a few states that psychologists are allowed to prescribe medication. However, this is not common practice everywhere.
Overall, psychiatrists differ from psychologists based on their training, education and the endings of their names. In psychiatry, the ending means treatment. So psychiatry means treatment of mental illness. For psychology, the ending means the study of. So psychology means the study of the mind and human behavior.
Hope this helps clarify any confusion.
A psychiatrist has a medical degree followed by a residency in psychiatry. Very few psychiatrists engage in talking therapy in the 21st century; most prescribe psychoactive medications for their patients and monitor therapeutic doses of those medications with laboratory orders (blood work).
A licensed psychologist has either a Ph.D. or a Psy.D. in clinical psychology. The amount of time required to complete a doctorate in psychology is as long, or even longer, than the completion of a medical degree with a specialization in psychiatry. Clinical psychologists use a variety of therapeutic interventions to help their patients. In three states in the USA, licensed psychologists can indeed prescribe psychoactive medications for their patients, so long as they've completed post-graduate training...
I strongly recommend that you "poke around" at the web-site of the American Psychological Association (www.apa.org) to gain more information about the discipline of psychology. Only about one-third of psychologists are clinicians, i.e., treat patients. Psychology is comprised of many sub-disciplines and specialties. My specialty, Industrial-Organizational Psychology, is the application of psychology to the world of work! We are interested in improving the organizations in which we work by engaging in a number of activities such as staff training, leadership development, occupational testing.
At the very simplest level, a psychiatrist can prescribe drugs for treatment programs and a psychologist can't. So I think a psychiatrist has more medical training, in addition to training on human behavior and the mind, than is required for a psychologist. Hopefully someone more informed will add to this, though :-)
A psychiatrist is an MD with additional training in psychiatry and a psychologist has a Ph.D. or Psy.D. from a non-medical program. The psychiatrists can prescribe medications and a psychologist generally cannot.