A psychiatrist is a doctor who has attended medical school and done a residency in psychiatry. In many states, psychiatrists and other medical doctors are the only professionals who can prescribe psychiatric medications. Many psychiatrists focus on severe issues such as major depressive disorder, bipolar disorder, and schizophrenia. These challenges usually require a combination of therapy and medication.

A clinical psychologist is a professional who has earned a PhD or a PsyD in clinical psychology. In most states, psychologists cannot prescribe medication, but are trained to work with individuals who have either mild or severe psychological issues. Some psychologists do not work as therapists, but instead teach at universities or conduct psychological research. The training program of PhD clinical psychologists has more of a research focus than the newer PsyD degree.

Therapists, clinical social workers, and marriage and family therapists also provide psychotherapy services in most states. Many social workers focus on solving social problems and connecting clients with appropriate resources. Marriage and family therapists tend to specialize on interpersonal and family dynamics. 

Psychiatric Nurses and Nurse Practitioners often work with doctors and can prescribe medication in some states.

Counselors are generally Master's-level psychotherapists who are focused exclusively on providing therapy to individuals, couples, or families. Counselors do not prescribe medications. Many counselors focus on common life issues, including stress and anxiety, mild to moderate depression, relationship conflicts, and work or career development. Although counseling graduate programs include a great deal of psychology, the emphasis is more on working with "problems of normal living" rather than severe mental illnesses. Licensed Professional Counselors (LPC's) have completed an accredited graduate program plus at least 2000 hours of supervised psychotherapy experience.

A life coach is not any of the above.  Coaches DO NOT diagnose.  Their technique focus on the present and the future and require clients to do "homework" in between sessions to work on behaviors that will move them towards feeling the way they want to feel; stress-free, energized, healthy, etc.