A physiatrist is a physician. But:
A physiatrist is to an orthopedic surgeon what a cardiologist is to a cardiac surgeon. The physiatrist works on bone and joint problems that don't need surgery but still need treatment and rehab from a physician before seeing a physical therapist.
However, you don't hear a lot about them because they tend to work Behind the Scenes of orthopedic practices or they practice under the moniker of a pain managment practice.
They won't hang their own shingle like an orthopedic surgeon or cardiologist because generally they have zero walk-in business, most of their business comes from referrals and they do better when attached to facilities such as rehab centers or orthopedic partnerships.
If you visited an orthopedic surgeon and he/she said you don't need surgery but you need some form of procedure and rehab, then he/she moved you along to another member of his practice, chances are you dealt with a physiatrist.
They work on things like connective tissue, bone and muscle, even some levels of nerve damage, that doesn't need surgery. Just like orthopedic surgeons, they can specialize in body parts and regions such as hands, lower body, feet, back, pediatrics, sports, etc.
Then, they'd be the ones who are more apt to address problems with medications rather than knives.
Not all hospitals have them, not all medical schools train them. But if you go to a private rehab center somewhere, you can bet there's a couple overseeing the activities. Also, sports teams use physiatrists a lot because they deal more with immediacy of joint and bone problems while orthopedic surgeons are a season-ending resort.
Finally, if you're obsessed with fitness and performance issues and study, and you're going to medical school, and you like dealing with athletes, this is the top of the food chain above physical therapists, certified trainers and all the rest.