REST API, which stands for Representational State Transfer Application Programming Interface, is an architectural style for designing networked applications. It is not a specific technology or protocol but rather a set of constraints and principles that define how web services should be structured and how they should behave. RESTful APIs are designed to be simple, scalable, and easy to understand, making them a popular choice for building web services and APIs on the internet.

Key principles and characteristics of RESTful APIs include:

1. Statelessness: RESTful APIs are stateless, meaning each request from a client to the server must contain all the information needed to understand and fulfill that request. The server does not store any client state between requests. This makes it easy to scale and distribute the service since each request can be handled independently.

2. Resources: Resources are the core abstractions in RESTful APIs. Resources are identified by URIs (Uniform Resource Identifiers), and each resource represents an object or concept, such as a user, product, or document. Resources can have multiple representations, such as JSON or XML.

3. HTTP Methods: RESTful APIs use HTTP methods (GET, POST, PUT, DELETE, etc.) to perform actions on resources. Each HTTP method has a specific meaning:

   • GET: Retrieve data from the server.
   • POST: Create a new resource on the server.
   • PUT: Update an existing resource on the server.
   • DELETE: Remove a resource from the server.

4. CRUD Operations: RESTful APIs often map CRUD (Create, Read, Update, Delete) operations to HTTP methods. For example, GET for reading, POST for creating, PUT for updating, and DELETE for deleting resources.

5. Uniform Interface: RESTful APIs have a uniform and consistent interface, making it easier for clients to understand and interact with them. This uniformity includes using standard HTTP methods, resource URIs, and status codes.

6. Stateless Communication: Communication between clients and servers is stateless, meaning each request/response cycle is independent. Clients must include all necessary information in each request.

7. Response Codes: HTTP status codes are used to indicate the outcome of a request (e.g., 200 for success, 404 for not found, 500 for server error).

8. Representation: Resources can have multiple representations (e.g., JSON, XML, HTML), and clients can specify their preferred representation using HTTP headers (e.g., Accept header).

9. Layered System: RESTful systems are often designed to be layered, with multiple components (e.g., load balancers, caches) working together. Clients are typically unaware of the underlying architecture.

10. Self-Descriptive Messages: RESTful APIs should use self-descriptive messages. This means that the information needed to understand and process a request or response should be included within the message itself or made discoverable through well-defined conventions.

RESTful APIs are widely used for building web services due to their simplicity and compatibility with HTTP, which is the underlying protocol of the World Wide Web. They are commonly used in web and mobile application development to facilitate data exchange between clients and servers over the internet.