They are, however distinct from dolphins, although the word "porpoise" is often used to refer to any small dolphin, especially in North America. A key difference is the shape of the teeth, which in dolphins are conical, whereas porpoises have spade-shaped teeth.
Porpoises tend to be smaller but stouter than dolphins. They have small, rounded heads and no beaks. In addition, a porpoise's dorsal fin is generally triangular, rather than curved like that of many dolphins.
Porpoises tend to be less acrobatic and more wary than dolphins; several species are under threat from man's activities.