What is the difference between a beak and a bill?

A beak and a bill – mostly in birds – is used for eating. There are two parts of a beak and a bill: 1) upper mandible (jaw), which is also called a maxilla, and 2) lower mandible (jaw), which is called mandible. 

The beak and the bill are similar and it is considered acceptable to use the beak and the bill to mean the same thing. However, there is slight difference.