API (Application Programming Interface) .

1. What is API? 

In computer programming, an application programming interface (API) is a set of subroutine definitions, protocols, and tools for building application software.

In general terms, it is a set of clearly defined methods of communication between various software components.A good API makes it easier to develop a computer program by providing all the building blocks, which are then put together by the programmer.

Popular API Examples

Broadly speaking, APIs make it possible for organizations to open their backend data and functionality for reuse in new application services. Google has APIs for search, calendars, translations, etc. Facebook and Twitter have APIs that allow software to automatically post status updates. Moreover, APIs have become so valuable that they comprise a large part of many business' revenues. Major web development companies like Google, eBay and Amazon are just a few of the companies that make money from their APIs.

okay to speak plainly an API is the messenger that takes requests and tells a system what you want to do and then returns the response back to you. 

To give you a familiar example, think of an API as a we're in a restaurant imagine you're sitting at the table with a menu of choices to order from and the kitchen is the part of the system which will prepare your order what's missing is the critical link to communicate your order to the kitchen and deliver your food back to your table that's where the waiter or API comes in.

the waiter is the messenger that takes your request or order and tells the system, in this case, the kitchen what to do and then delivers
the response back to you in this case food.