Dolphins are mammals. They are any of the toothed whales belonging to the mammal family Delphinidae (oceanic dolphins) as well as the families Platanistidae and Iniidae, the two that contain the river dolphins. Of the nearly 40 species of dolphins in the Delphinidae, 6 are commonly called whales, including the killer whale and the pilot whale. The name dolphin is also applied to members of the fish genus Coryphaena (family Coryphaenidae). Most dolphins are small, measuring less than 10 feet in length, and have spindle-shaped bodies, beaklike snouts (rostrums), and simple needle-like teeth.

Source: Britannica