What is the difference between psychiatrists, psychologists, counselors and family therapists?

Medications. Of all that you mentioned, except for very rare exceptions, psychiatrists are the only ones with a medical degree who can prescribe medications. The other differences have to do with the formal training a person received, e.g. Graduate school in psychology through a university vs school of social work. In practice, the services they provide often overlap.
Therapy vs meds. Speaking very generally, most psychologists hold either a PhD or a PsyD degree (I hold both) and practice psychotherapy. Psychiatrists hold either an MD or a DO degree, and they prescribe psychotropic medications, and generally do not do all that much therapy these days, but years ago psychiatrists would do both. Social workers, psychiatrists, and psychologists all do counseling/psychotherapy.