A Representational State Transfer Application Programming Interface (REST API) is a set of rules that allows different software applications to communicate over the internet. It uses standard HTTP methods like GET, POST, PUT, and DELETE to perform operations on resources, which are identified by unique URLs. This makes it easy for developers to access and manipulate data in a consistent way. REST APIs are designed to be stateless, meaning each request from a client contains all the information needed for the server to fulfill it. This simplicity and scalability make REST APIs popular for building web services, enabling seamless integration between different systems and applications.