What difference between ocean and sea?

The Word Ocean refers to one of the Earth's four distinct large bodies of water. The Atlantic, Pacific, Indian and Arctic Oceans. It can mean all the water that covers the Planet, and derives from the Greek Okeanos. The Word sea can mean a vast body of water, but generally refers to bodies of salt water smaller than the great oceans that are generally marked off by land boundaries or enclosed by land. (The North Sea or Caspian Sea for example