A REST (Representational State Transfer) API (Application Programming Interface) is a set of rules and conventions for building web services. It's a way for different software systems to communicate with each other over the internet, using the HTTP protocol. REST APIs are stateless, meaning each request from a client to a server must contain all the information needed to understand and process the request. They are also cacheable, meaning responses can be stored for later use, which can improve performance. REST APIs use resources, which are any kind of object, data, or service that can be accessed by the client. A resource could be a user, a photo, a document, or any other type of data that an application might need to handle. REST APIs use HTTP methods to perform operations on these resources. The most common methods are GET (retrieve a resource), POST (create a resource), PUT (update a resource), and DELETE (remove a resource). REST APIs are widely used in web development because they are simple, flexible, and easy to understand. They can be used with any programming language or platform, and they can handle a wide variety of data types and structures.