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What is an API
An application programming interface (API) is a computing interface which defines interactions between multiple software intermediaries. It defines the kinds of calls or requests that can be made, how to make them, the data formats that should be used, the conventions to follow, etc.
It can also provide extension mechanisms so that users can extend existing functionality in various ways and to varying degrees.
An API can be entirely custom, specific to a component, or it can be designed based on an industry-standard to ensure interoperability. Through information hiding, APIs enable modular programming, which allows users to use the interface independently of the implementation.
In building applications, an API simplifies programming by abstracting the underlying implementation and only exposing objects or actions the developer needs.
While a graphical interface for an email client might provide a user with a button that performs all the steps for fetching and highlighting new emails, an API for file input/output might give the developer a function that copies a file from one location to another without requiring that the developer understand the file system operations occurring behind the scenes.