Scientist recognize more than 70 species of whales. Collectively known as cetaceans, whales are further classified into two scientific groups called toothed whales (odontocetes) and baleen whales (mysticetes). These two groups of whales differ from each other in several ways.

Toothed whales are named for their simple, peglike teeth, which vary considerably in number and size among the species. The teeth of dolphins are conical and interlocking, while those of porpoises are spade-shaped. River dolphins have numerous teeth; most beaked whales have only one or two visible pairs. Teeth are adapted for grasping and tearing, rather than chewing.

Baleen whales have no teeth. They have stiff, fringed plates of baleen that hang down from the upper jaw. Baleen is composed of keratin, a protein compound that also makes up human hair and fingernails. Baleen whales are filter feeders. They take in huge mouthfuls of water containing small fishes or invertebrates. The baleen traps the prey, and water is forced back out of the mouth.

Toothed whales and baleen whales can be distinguished in other ways, as well. Toothed whales have a single blowhole, while baleen whales have two. In general, baleen whales are much larger than toothed whales, ranging in length from about 6.4 to 27 m (21-85 ft.). Most toothed whales are less than 6.1 m (20 ft.) long.