Nearly everybody has to do an internship in medicine. If you're planning on moving on to something else after your internship year (surgery, rads, OB, etc), you apply for a "preliminary" position - which means you're not promised a spot after PGY-1 year and they don't expect to see you back (you can apply to stay if you change your mind). Once your internship is done in a "preliminary" position, you may well move on to a different program at a different hospital for PGY-2 and beyond. If you want to do your residency in Internal Medicine or a subspecialty, you apply for a "categorical" position - which means you're going to stay there for three years.