The Pinnipeds ("fin-feet", lit. "winged feet") can be divided into three families:
the Phocidae (true seals),
the Otariidae (eared seals),
and the Odobenidae (walruses).

The true seals, to which the common seal and the grey seal belong, are divided into two subfamilies: the Phocinae (10 species), which mainly exist in the arctic and sub arctic areas, and the Monachinae (9 species, including one extinct species), which exist in the subtropical areas and around the Antarctic.
The true seal can be recognised by its streamlined body and through its lack of external ears. In the water they use their hind flippers to move along, which makes their style of swimming look rather like that of a large fish. They move clumsily on land, wriggling along by alternately moving their chest and their pelvis from side to side. Some species are like caterpillars in motion and others move like snakes.