What's an API?
To understand application interfaces, it is important to first understand application programming interfaces (APIs). APIs are well-defined mechanisms that are built to connect to a resource, such as an application server, middleware layer, or database. APIs allow developers to invoke the services of these entities to obtain some value from them. In other words, an API is just a set of procedures that can be called from outside the app to get it to do something.
Programmers write APIs. Therefore, it should come as no surprise that APIs are as unique (and sometimes quirky) as the people who write them. The best APIs can access real-time data, such as information gathered from the floor of an industrial plant, or can draw information residing on mainframes.