Zoologists say it's a mammal because it has fur and is warm-blooded. Sea lions look like seals, but are really only distant relatives, like third cousins. You can tell them apart by their earlobes. Sea lions have little earflaps on their heads like earlobes.
Seals just have holes in their head that lead to their ears.
Another way to tell them apart is to challenge them to a road race. It works O.K., but it's a little slow. Like all pinnipeds, sea lions eat fish, shrimp, squid, and other animals which they chase out of the water, grab with their teeth, and swallow whole. Their table manners aren't so good, but they are good at making a lot of blubber which keeps them warm when swimming in the cold ocean water. Blubber is a thick layer of fat under their skin.