There's a good reason seals and sea lions look so similar—they're both members of the pinniped taxonomic group, a name which refers in Latin to their "fin feet." Walruses are also a part of the clade but while their prominent tusks set them apart, seals and sea lions can be slightly harder to distinguish.
The easiest way to tell the two marine mammals apart is a matter of ears—or lack thereof. While sea lions have small ear flaps, seals have just tiny holes on the sides of their sleek, aquadynamic heads. This telltale sign is one of several adaptive differences based on slightly different lifestyles.
Seals are typically loners, who spend most of their life in the water. Their stubby front flipper-like feet and backward-facing hind flippers are little help on land, where they must wriggle about to move, but make them quick and agile underwater.
Meanwhile, sea lions are larger, louder and more social, often gathering in herds of up to 1500 individuals barking to each other. Accordingly, their feet are well-suited to lugging their several hundred pound bodies around rocky beaches with elongated fore flippers and hind flippers that are able to rotate forward.