On the surface they look very similar but there are some key differences to tell these animals apart.
Seals and sea lions are members of a group of mammals known as pinnipeds. There are three families within the group: true seals, eared seals, and walruses. True seals include species like the grey, spotted, elephants, and monk seals, while eared seals are not really seals at all, and include sea lions and fur seals. As the name suggests, one of the main differences between the two families is their ears, True seals have no external ears, just one tiny hole on each side of their heads, while eared seals have small ear flaps. However, there is a simpler way to tell seals and sea lions apart; they have completely different body shapes and ways of moving. True seals are much better adapted for swimming than they are for moving around on land; their short arms and inflexible tails make them clumsy out of the water, but beneath the waves they move gracefully, using their front limbs to steer. Eared seals have longer front limbs, and can curl their back flippers underneath their bodies. They use their front fins to pull themselves through the water, and on land, they can walk on their limbs quite effectively.
This video also explains the differences: