What Is The Difference Between An Ocean, A Sea, A Bay And A Gulf?

Ocean is an open expanse of salt water usually between continents or geographic regions. (think Atlantic, Pacific). It represents a far larger body of open water than a sea. 

Sea can be either sweet or salt water but needs a connection to an ocean, usually via a river. (think Black Sea). The phrase sea can also be used in a local/regional context as a part of a larger body of water. In general, a sea is defined as a portion of the ocean that is partly surrounded by land.

Bay is body of water which is partially surrounded by land with a wide mouth which opens into ocean. A bay is usually smaller and less enclosed than a gulf. water bodies landlocked with three sides and have wide discharge into ocean or lake or river.e.g bay of Bengal,Hudson bay etc. 

it is larger than bay,but have narrow discharge section into ocean. E.g gulf of Mexico ,Persian gulf etc. gulf is a deep inlet of the sea with a narrow mouth, which is almost surrounded by land.Gulfs vary greatly in size, shape, and depth. They are generally larger and more deeply indented than bays.