An Open API (often referred to as a Public API) is a publicly available application programming interface that provides developers with programmatic access to a proprietary software application. APIs are sets of requirements that govern how one application can communicate and interact with another. They also allow developers to access certain internal functions of a program. In the simplest terms, an API allows one piece of software to interact with another piece of software. They may be used by both developers inside the organisation that published the API or by any developers outside that organisation who wish to register for access to the interface.