After a physician completes four years of medical school, their training is not over. They must then complete an internship (also called PGY1 or Post Graduate Year 1) and then a residency (also termed Graduate Medical Education or GME). An internship typically lasts one year and a residency can last from three to seven years depending on the specialty.
Military GME is not much different from civilian GME in that virtually all specialties and sub-specialties are represented. Joining the military will not limit the specialties from which a medical student may choose.