Psychiatrists are medical doctors and psychologists are not.  Psychiatrists treat mental issues such as bipolar disorder and schizophrenia with medicine (i.e. pills).   Psychologists use counseling and/or therapy to deal with mental health issues. The suffix "-iatry" means "medical treatment," and "-logy" means "science" or "theory." So psychiatry is the medical treatment of the psyche, and psychology is the science of the psyche.