A REST (Representational State Transfer) API is a popular type of web API (Application Programming Interface) that allows client applications to access and manipulate data on a web server using a set of well-defined HTTP requests. REST APIs are based on a set of architectural principles that emphasize simplicity, scalability, and modularity, making them a popular choice for building web services.

A key feature of REST APIs is their use of standard HTTP requests such as GET, POST, PUT, and DELETE to interact with resources on a server. REST APIs typically use JSON (JavaScript Object Notation) or XML (eXtensible Markup Language) to format data, making it easy for client applications to parse and use.