A "chop" is simply a small cut of lamb, pork or veal, cut from the back. Rack of lamb is a piece of the back from the rib section, it can be cut into chops and cooked, or roasted whole and then sliced; it's the lamb equivalent of ribeye steak or prime rib, respectively. There are also loin chops and shoulder chops, but these aren't nearly as nice; I doubt that most restaurants would serve a shoulder chop.