API stands for Application Programming Interface.
An API is fundamental to modern programming techniques. It is generally an auxiliary file that is used when writing an application program (apps). This file is linked to the program directly when it is compiled or dynamically when the program is run. This is often used to provide a consistent way to manage subroutines and is used in all kinds of programs. An API consists of the software itself, a defined method of calling the program, and rules for its use.
The calling program supplies some specific information about what it wants and the API returns the translated answer for the information.
Consistency is one of the main advantages of an API. In addition it can provide a way for an application programmer not to have to know the intricate details of some special format like, for example, ePub.
An API is often part of an SDK, Software Development Kit, that includes other items to help the programmer.
Some companies specialize is writing API's. They will often have a program that they have written to make use of the API. For example Adobe Digital Editions provides an API that users can utilize to provide access to the ePub file itself and they can also provide the mechanism to support DRM on the file.