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.^[1] A computer programmer uses the API to make computer applications.

An API works by making a structure for computers to interact with each other, including rules for interactions and methods the computers can use to quickly send data to each other.^[2] Because an API's goal is to connect computers with other computers, they are different from a user interface, which has the goal of connecting human users with computers.

Main types of web APIs:^[3]

• 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.