Basically, a standing rib roast is the prime rib portion still on the bone. There are fatty pockets close to the bone, so your standing rib roasts will have a fattier content and better marbling which leads to a better flavor. If you can get your hands on a standing rib roast, you can ask the butcher to cut away the bones which are a) a wonderful rack for your prime rib roast and b) bones for making a superb stock for your leftover prime rib soups.