Dolphins, unlike most creatures living in the sea, are mammals. They are warm-blooded and nurse their young with milk from mammary glands. They are part of a group of mammals called cetaceans which includes all whales, dolphins and porpoises.
Cetaceans are divided into two suborders, Mysticeti and Odontoceti. The Mysticeti suborder includes whales which have baleen plates rather than teeth. They feed by trapping krill (tiny crustaceans or fish) inside the baleen in their mouth. The largest whale of all, the blue whale, is in this suborder.
Toothed whales, dolphins, and porpoises are in the suborder Odontoceti. They are generally smaller than the whales of the Mysticeti suborder, with the exception of the Sperm Whale, which can be around 18 metres long and is a mighty deep water hunter. Orcas (Killer Whales) are also in this suborder. Some species live in the shallow waters close to shore, some live exclusively in deep oceans, while others are found only in rivers.
There is little obvious difference between dolphins and porpoises. The simplest way to differentiate them is to look at their teeth. Dolphin teeth are round in cross-section whereas porpoise teeth are flat.