RESTful API, also known as REST API, is a software architectural style. An API is an application programming interface, which is a code of instructions on how two or more software programs work together and interface with one another. When a computer user brings over information from one application to another, they are interacting with a set of API protocols.
RESTful APIs take advantage of existing computer protocols by not requiring developers to add additional software or install libraries. In this manner, a programmer can change or refine a program using the API building blocks. When these are used for web application APIs, a RESTful API works to integrate with HTTP protocol. The term REST is an acronym for representational state transfer and refers to a software architecture for distributed hypermedia systems.