Whales, dolphins and porpoises are the commonly used terms to describe animals of the order Cetacea. Cetacea is the scientific classification. Cetaceans are mammals, which means they are endothermic (produce body heat from within), give birth to well-developed young, have fur at some point in their life cycle (dolphins are born with a few bristly hairs on their chin), and females nurse their young. People are mammals, as are giraffes, elephants, raccoons and rabbits. Cetaceans are entirely aquatic, finding food, mating, giving birth and raising their young all in the water.

Cetaceans are extremely well-adapted to aquatic life. They have a streamlined body with an elongated rostrum for easy swimming, flukes (tail fin) to propel them through the water; a dorsal fin to stabilize them in the water, nostrils which have migrated to the top of their head to become the blowhole, and pectoral fins to aid in steering. In addition to these morphological adaptations, they are physiologically adapted to dive to great depths for long periods of time. There is a great diversity among the 76 species of cetaceans, but the basic external anatomy is shown below.

Cetaceans are divided into two main groups, or suborders: the toothed whales (Odontoceti) and the baleen whales (Mysticeti). Toothed whales include dolphins, porpoises, and sperm whales. They have teeth and usually catch their prey one at a time. Baleen whales are filter feeders; they strain the water using strong, broom-like plates that hang down from their upper jaw. These baleen plates are made of keratin, like your finger nails. Baleen whales include humpback whales, minke whales, the largest animal to ever live, the blue whale and what is now the rarest whale, the right whale.

The toothed whale suborder can be divided into 6 distinct families with about 65 different species noted. Most toothed whales do, in fact, have teeth. The narwhal (Monodon monoceros), for example, has just one visible tooth: the 10-foot long tusk seen in most males. The sperm whale (Physeter macrocephalus) has teeth only on the bottom jaw. The upper jaw is equipped with indentations where the lower teeth fit to secure a grip on their prey. The more familiar members of the dolphin family have more than 100 teeth on both the upper and lower jaw. One noticeable feature of dolphin dentition is that all the teeth are the same! This is called homodont dentition.