What is the difference between an ocean and a sea?

Seas are smaller than oceans. Seas are usually a part of an ocean. Seas are close to land so they are shallower than oceans.

Check out this website:
http://www.differencebetween.net/science/difference-between-ocean-and-sea/

Hope this helps (:

But wikipedia says:

A sea is a large expanse of saline water connected with an ocean, or a large, usually saline, lake that lacks a natural outlet such as the Caspian Sea and the Dead Sea. The Sea of Galilee is a small freshwater lake with a natural outlet, but the term was applied to it anyway. The term is used colloquially as synonymous with ocean, as in the tropical sea or down to the sea shore, or even sea water referring to water of the ocean. Large lakes are sometimes referred to as inland seas, such as the Great Lakes. Many seas are marginal seas. Check here http://www.surf-school-spain.com/en/.