APIs are series subroutine definitions, communication protocols, and tools for building software. The most important aspects of an API are the functionalities that it offers, combined with its documentation, which facilitates the user in the usage and implementation of the software itself in another software. An API can be the interface that allows an application software to use a system software.

An API usually has a specific release policy that is meant to be used by a specific group of recipients. This can be the following:

• Private and for internal use only
• Partner and usable by determined groups only—this may include companies that want to integrate the service with theirs
• Public and available for every user