A RESTful API (Representational State Transfer) is a web service that adheres to the principles of REST architecture. It operates over HTTP and allows interaction with resources using standard methods such as GET, POST, PUT, and DELETE. Each resource is identified by a unique URI, and responses are typically in JSON or XML. RESTful APIs are stateless, meaning each request from a client must contain all the information needed to understand and process the request. This makes them scalable and easy to work with for developers.