They are actually a bit of the same thing. Round roasts come from the rear of the cow and are sold in the store as top round or bottom round. The top round is lean and more tender than the bottom round. The bottom round is divided into two parts, the bottom round and a rump roast. The naming of the different cuts can be quite confusing so if you can't find it by the name bottom round roast, ask your butcher or someone at the meat counter for some assistance.