The Catalogue of Life lists 37 species in the family Delphinidae, the taxon that holds all oceanic dolphins (fresh-water dolphins are found elsewhere). The common names of some species in this group can be misleading with a handful of them often referred to as whales, including the well-known Orca or Killer whale. However, although dolphins, porpoises, and whales all belong to the order Cetacea the Delphinidae are united by a number of shared characteristics including a single blowhole, streamlined bodies (ie wide in the middle and narrow at each end), and a beak-like nose.
The majority of species have been assessed by the Catalogue of Life's partner the IUCNRedlist for conservation status, with a few currently classified as Vulnerable or Near Threatened and the species Cephalorhynchus hectori listed as Endangered.
Did you know that Dolphins, because of their excellent hearing, sonar capabilities and underwater vision, have been used by the US Navy for decades to locate things in the sea. Once they have undergone two years of training they are ready to embark on a mission such as mine-sweeping off the coast of Croatia.
The etymology of the name dolphin is interesting, having very little variation in most languages. To hear a in-depth, inspired and entertaining overview of its origins listen to this pod cast from the lively Dolphin Communication Project.