What is an API?

An application programming interface (API) is a language and message format that a computer system, library or application provides in order to allow requests for service to be made of it by other computer programs, and/or to allow data to be exchanged between them. APIs exist for windowing systems, file systems, database systems, and, of course, networking systems. APIs are implemented by writing function calls in a program, which provide a linkage to the required subroutine for execution.