The most spectacular cetaceans are, arguably, the really big ones - sperm whales, blue whales, and so on. However, while our knowledge of the exact numbers is a little hazy, somewhere around half of the cetacean species alive today represent much smaller animals. The majority of these belong to just two taxonomic families.
By far the most numerous are the "true" or "oceanic" dolphins, a family that also includes killer whales and pilot whales - small in comparison with the like of humpbacks, but fairly large by most standards. The second family are the porpoises, which are exclusively small, by cetacean standards, and usually slightly smaller than dolphins.
But there are a few small-sized cetacean species that fall into neither group. These oddities share one thing in common: they don't live in the sea. While they are, of course, just as fully aquatic as their better-known kin, this has lead to them receiving the common collective name of "river dolphin", thus distinguishing them from the "oceanic" sort that most people are more familiar with.