What are Seals, Sea Lions, and Walruses?

They're the fin-footed ones. The scientific name for the group of animals that includes seals, sea lions, and walruses is Pinnipedia. It's a Latin word meaning "fin-footed" and refers to the modification of limbs to flippers. Pinnipeds have four flippers - one pair in front, and one pair in back.
They're mammals.

Pinnipeds share five common characteristics with other mammals. They're warm-blooded (maintain a high and constant body temperature independent of the surroundings), give live birth, nurse their young, breathe air, and have hair. Since they live in the marine environment and they find their food at sea, pinnipeds are called marine mammals. Other marine mammals include whales and sea otters.

They're adapted for land and sea. Although most of their lives are spent in the water, pinnipeds, unlike whales and dolphins, are also dependent on land. Pinnipeds may come ashore periodically to rest and bask in the sun, and at least once each year, during their breeding season, most species congregate on beaches or sea ice to give birth and breed.

http://www.seaworld.org/Pinnipeds/whatarethey.html