REST API (Representational State Transfer) is an architectural style for web services.
REST uses standard HTTP methods to perform operations on resources.

HTTP Methods:
- GET: retrieve/read resource. No request body.
- POST: create new resource. Data sent in request body.
- PUT: update existing resource completely.
- PATCH: partially update a resource.
- DELETE: remove a resource.

HTTP Status Codes:
- 200 OK: request succeeded.
- 201 Created: new resource created successfully.
- 400 Bad Request: invalid request from client.
- 401 Unauthorized: authentication required.
- 403 Forbidden: authenticated but not authorized.
- 404 Not Found: resource does not exist.
- 500 Internal Server Error: server-side error.

REST principles: Stateless, Client-Server, Cacheable, Uniform Interface.
JSON is the most common data format for REST API responses.