Application programming interface

An application programming interface (API) is a set of functions, procedures, methods or classes used by computer programs to request services from the operating system, software libraries or any other service providers running on the computer. A computer programmer uses the API to make application programs.

An API works by communicating with and exchanging data with other systems, acting as a messenger between the user and the system to retrieve the necessary data the user is requesting from the system.

Main types of web APIs:

• Open APIs - available to the public; they can be accessed by any external users.
• Partner APIs - available to strategic business partners; they are exposed to a public API developer portal.
• Internal APIs - available to a company's internal development teams; they are exposed to a private API developer portal.
• Composite APIs - are a sequence of tasks bundled into a single API call.

Types of API include web services API like the Twitter API, which allows programs to use the API to receive updates on tweets.

Application Programming Interface Media

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  Screenshot of web API documentation written by NASA

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  A diagram from 1978 proposing the expansion of the idea of the API to become a general programming interface, beyond application programs alone

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  Although the people who coined the term API were implementing software on a Univac 1108, the goal of their API was to make hardware independent programs possible.