Brontosaurus is a nomen nudum, its correct name is apatosaurus. The catchier name brontosaurus ("thunder lizard") is still more commonly known.
It was related to the larger diplodocus, but brontosaurus had an overall more heavier build than diplodocus. Brontosaurus had a long flexible tail that had 82 vertebrae and was thus langer than diplodocus' tail in fact it is one of the longest tails known. The extremely flexible tail could be used as whip to fend off attackers.