What's the difference between an orthopedic surgeon and a podiatrist?

Several. An orthopedist is an md who has done a residency in orthopedics. A podiatrist is not an md but a dpm, after college has attended 4 years of podiatry school and then done a residency for 1-4 years.
Scope of practice. Orthopedic surgeons are trained to treat any your entire musculoskeletal system, while podiatrists are specialize in the treatment of the foot an ankle. In many hospitals the resident train under the same surgeons.
Feet. Poditrists only work on feet but not all perform surgery. Orthopedists work on the entire body and some specialize in feet.