MD - diagnoses and decides on a course of treatment that may include PT.
Therapist treats using the Dx MD has given and the treatment parameters defined by the doc (for most docs, they don't - it's just "eval & treat." In PM&R, we give specific instructions). They personally apply modalities like heat, ultrasound and e-stim, help the patient to learn a stretching and strengthening program, work with the patient hands-on for 1/2 - 1 hour 2-3 times a week on average.