What is the difference between a sea and a ocean?

These are all Words that shade into each other in meaning. Ocean is usually bigger than sea and a channel is usually fairly narrow between two land masses, for instance 'The English Channel' (look it up on a map amnd you will see what I mean). However, some sea areas have been called oceans or seas according to the fashion of the times. For instance 'The North Sea' was known as 'The German Ocean' for a long time.