There are about 40 species of marine dolphins (family Delphinidae) and porpoises (family Phocoenidae), combined. The chief differences between dolphins and porpoises are in their snouts and teeth. A true dolphin has a beak-like snout and cone-shaped teeth. A true porpoise has a rounded snout and flat or spade-shaped teeth.
Sources: Macmillan Illustrated Animal Encyclopedia, pp. 112, 114; Nowak, Ronald M. Walker's Mammals of the World, 5th ed., vol. 2, pp. 969-82.