Whales, dolphins and porpoises belong to the order Cetacea. These are aquatic mammals that have streamlined bodies highly evolved for swimming. Their hind limbs have become vestigial as part of this streamlining. All whales, dolphins and porpoises are cetaceans, including the biggest animal ever to have lived - the blue whale.
Scientific name: Cetacea
A comparison of whale and dolphin size in relation to humans - from the 30m long blue whale to the 2.5m long harbour porpoise.