A REST API, or Representational State Transfer Application Programming Interface, 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 typically represented in formats like JSON or XML. This makes it easy for developers to access and manipulate data from a server. 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, allowing different systems to work together seamlessly.