api-key

I'm seeing this word in almost every service application.. What is an API KEY really and what are its uses? How does it really work? Also please, if possible, explain the difference between public and private API KEYs.

I have 2 api key, one with restriction for android apps and another without, when I put the app to use the api with restricti ... ithout restrictions everything works normally. Already check the SHA - 1 and the package and everything seems this correct.