Top round is a beef roast cut from the inside of the round, or the rear leg of cattle, while bottom round is cut from the outside of the round. Both are lean and muscular.
According to Chow.com, the round primal "is placed on the butcher’s block with the outside on the bottom and the inside on the top, so these cuts became known as bottom and top round."
A major difference is that top round is more tender than bottom round.
Both most often are cooked slowly in a moist atmosphere, such as in a pot roast or stew. Top and bottom round are both used for deli-style roast beef.
Each is sometimes cut into steaks. Chow.com notes that, next to the sirloin, the top round steak cut is the most tender and expensive of the round steaks.
Top round is found whole or separated into halves, according to Chow.com, while bottom round is cut into three sections: the eye of round, the flat and the heel.