Q: What is a RESTful API?
A: Although this isn't a typical Windows FAQ, it's common to see various services that support "RESTful APIs"—and many people have no idea what this really means.
REST stands for Representational State Transfer, which consists of executing different operations on a system via the standard HTTP methods (verbs) GET, POST, PUT, and DELETE. A REST API is a set of operations that can be invoked via HTTP (or HTTPS) using these four standard verbs; the parameters for the operations are passed as part of the URL. This standard approach makes REST APIs easy to interact with from almost any language, because all actions are simple HTTP requests. For example, GET can be used in a web browser to see results (or anything else that can communicate with HTTP).
System Center Orchestrator has a REST service that makes it easy to obtain information (e.g., to list all runbooks, using http://<Orchestrator Server>:81/Orchestrator2012/Orchestrator.svc/Runbooks). In addition, the REST service can be used to trigger Runbooks.
If a service offers a REST API or is RESTful, this simply means that the service is easy to interact with and that it uses the HTTP protocol with the four verbs (GET, POST, PUT, DELETE) for executing actions.