This document provides an introduction to REST APIs. It defines an API as a set of tools and protocols for building software. A REST API is an architectural style for web services that uses HTTP requests to GET, POST, PUT, and DELETE data. REST APIs have features like being simpler than SOAP, having documentation, and providing error messages. The core principles of REST are that it is stateless, uses a uniform interface, is layered, cacheable, and has code-on-demand. Common HTTP methods map to CRUD operations on resources. REST APIs offer advantages like scalability, flexibility, and independence.