Physicians are medical doctors who have been trained primarily to look after your body. They usually have hospital privileges and clinical appointment times which allow interview, examination, and specific testing to determine illness. After assessments are completed, doctors then treat the "patients" usually with medications. We all know how busy doctors can be and I have noticed signs in some of their offices informing patients that due to time restrictions, only one problem will be dealt with at a visit.
It is interesting to note that a large percentage of individuals who visit their doctor either don't have a physical ailment or are ill because of a mental or emotional problem. Relationship difficulties, grief, depression, anxiety, or other psychological problems can result in a visit to the doctor even if the problem might be treated better in another setting.
Psychiatrists are medical doctors who have specialized in thought and mood disorders. Some have hospital privileges which they use for testing or admitting purposes. Many operate a private clinic. They focus on assessment and treatment of individuals who may be suffering from a wide range of mental illness that include schizophrenia, sleep disorders, various types of depression, or other diagnoses most often referred to in the Diagnostic and Statistical Manual (DSM-IV-TR) published by the American Psychiatric Association. Most often an assessment consists of an interview with the psychiatrist who then offers a number of treatment options for the "patient" with follow-up appointments to assess their effectiveness.
Psychologists are professionals who study how people think, feel, and behave from a scientific viewpoint. They then apply the knowledge to help people understand, explain, and change their behaviours or develop healthier patterns. This is done through assessment, consultation, or treatment in specific areas of focus such as Industrial/Organizational, Clinical, Forensic, Educational, or Research Psychology. Psychologists may work in any of these specific settings or operate a private psychology practice where "clients" book and attend appointments. Psychologists do not usually have prescription or hospitalization privileges but instead focus on providing "talk therapy" to help their "clients" with difficulties.
Physicians, psychiatrists, and psychologists must have more than six years of formal post-secondary training as well as internships before they can practice independently. They are each licensed through regulatory bodies which have Code of Ethics and Standards of Practice. They all use science as a foundation to their work.
What differentiates them is how they help others. There is some overlap, but their focus is very specific to the field in which they practice.
If you are having some physical, mental or emotional difficulties, you may benefit from having a team of professionals who will examine your situation and work together with you in order to help you to reach your maximum health potential.