Dolphins, whales, and porpoises are members of a group of mammals called cetaceans. Dolphins and porpoises are very similar in appearance. Their chief differences occur in the snout and teeth. Dolphins have a beaklike snout and cone-shaped teeth. Porpoises have a rounded snout and flat or spade-shaped teeth.
Whales are much larger than most dolphins and porpoises.
Scientists apply the term dolphin to two families of cetaceans, marine dolphins and river dolphins. There are 32 species of marine dolphins. They are found in nearly all oceans, and most of them live only in salt water. Many species of marine dolphins remain near land for most of their lives, but some live in the open sea. River dolphins live in fresh or slightly salty water.