 Whale is the common name for a widely distributed and diverse group of fully aquatic placental marine mammals . They are an informal grouping within the infraorder Cetacea , usually excluding dolphins and porpoises . Whales , dolphins and porpoises belong to the order Cetartiodactyla with even @-@ toed ungulates and their closest living relatives are the hippopotamuses , having diverged about 40 million years ago . The two parvorders of whales , baleen whales ( Mysticeti ) and toothed whales ( Odontoceti ) , are thought to have split apart around 34 million years ago . The whales comprise eight extant families : Balaenopteridae ( the rorquals ) , Balaenidae ( right whales ) , Cetotheriidae ( the pygmy right whale ) , Eschrichtiidae ( the gray whale ) , Monodontidae ( belugas and narwhals ) , Physeteridae ( the sperm whale ) , Kogiidae ( the dwarf and pygmy sperm whale ) , and <unk> ( the beaked whales ) . 
