A clinical social worker is a person that has earned a masters degree in social work who has passed a test and been licensed by the State of California to practice psychotherapy.
A psychologist has doctorate degree (Ph.D. or Psy.D) in behaviorial sciences. They also do psychological testing as well as counseling.
A psychiatrist is a medical doctor who can prescribe medications. They generally focus on the medical aspect of healing through the use of medication, "antidepressants", mood stabilizers and anti-anxiety drugs. They are the only mental health professional that can prescribe medication.