What is the difference between a porpoise and a dolphin?

Dolphins and porpoises are cetaceans, or marine animals related to whales. Dolphins are members of the Delphinidae family and can grow to be up to 30 feet long. Porpoises of the Phocoenidae family are smaller and stouter, with further structural changes. Despite the fact that both species are highly intelligent, dolphins are thought to be more gregarious and live in bigger, more stable groups. Although the terms "dolphin" and "porpoise" are sometimes used interchangeably, the name "porpoise" should only apply to the Phocoenidae family, which contains seven porpoise species.

The fundamental anatomical distinctions between dolphins and porpoises are anatomical. Looking at the skull and dorsal fins, one can determine the difference between the two creatures. Dolphins often have a "beak" which gives them the appearance of being "long-nosed", but porpoises do not have a beak and so seem more "flat-faced". Dolphins' dorsal fins are normally curled or hooked, whereas porpoises are more triangular in form. Close inspection reveals that dolphins have cone-shaped teeth, whereas porpoises have flat or spade-shaped teeth.