What is a REST API?

First, it's helpful to understand the two parts of the term:

• API (Application Programming Interface): An API is a set of rules and definitions that allows two different software applications to communicate and exchange data with each other. It acts as an intermediary, processing requests and ensuring that the systems can interact seamlessly.
• REST (Representational State Transfer): REST is not a technology itself, but rather a popular and powerful "architectural style" or a set of design principles for building these APIs. When an API is built following these principles, it is described as being "RESTful."

Therefore, a REST API (or RESTful API) is an API that is designed according to the constraints of the REST architectural style.

This style is built upon the same proven technologies that power the World Wide Web, which makes REST APIs highly flexible, scalable, and easy for developers to work with. The communication happens in a simple client-server relationship: the client sends a request for information (a resource), and the server sends back a representation of that resource. This is all done using standard, well-understood web protocols (like HTTP).