There are over thirty different types of dolphin, ranging from the hefty Orca (or Killer Whale) to the tiny Hector's dolphin off the coast of New Zealand.
Dolphins are Mammals and Ceteceans (which include whales). Most dolphins are 'Delphinidaes' but river dolphins are 'Platanistidae'. Here are just a few of them:
For a full list of cetaceans click here