Some psychologists treat individuals with mental disorders and abnormal behavior. Those who do this are usually called clinical or counseling psychologists. To be a practicing clinical or counseling psychologist, you need to have a doctoral degree plus proper licensing. They acquired their MD by going to medical schools, instead of graduate schools. The difference is that after medical school, they have done a 3-year residency in psychiatry, where they learned to make a diagnosis and treat mental illness, normally by different medications.
Forensic psychology is another relatively new field of psychology.
The jobs of clinicians in this field differ, but all areas deal with some interaction of psychology and the law. Therefore, this field is often referred to as criminal psychology. There are many different areas in which forensic psychologists can practice. State mental hospitals, law enforcement agencies, and penal institutions are just a few examples of different areas in which forensic psychologists practice. The FBI is a main agency that employs forensic psychologists.