Seas are partly contiguous to a land mass and are also often in land (surrounded by land), like the famous Caspian and Aral Seas.
Oceans are enormous when compared to any sea. The largest sea in the world is the Mediterranean with a total area of 1,144,800 square miles which is much smaller than the smallest ocean in the world the Arctic at 5,427,000 square miles.
There are only five oceans in the world namely the Pacific, the Atlantic, the Indian, the Southern and the Arctic while there are dozens of seas around the world. Seas being close to land are generally much shallower than oceans and therefore it is possible for plant and animal life to thrive on a sea bed because it is generally lit up by light. Ocean beds are much much deeper, and support only very basic life forms like bacteria and shrimp that feed on them in isolated parts. This is because no light reaches there and the pressure are such that marine life as we popularly conceive cannot survive. Some idea of the depth of an ocean can be had from the fact that the Pacific Ocean has an average depth of 3790 meters or 12,430 feet. The average depth of the Mediterranean Sea on the other hand is 4690 feet.