Okay, so I'm a student new to REST. Yet it seems obvious. And simple.
I would have found it helpful to see description of what behavior would constitute NONrestful behavior. POSTs? POSTs that are used for navigation instead of submitting information?
Anti-REST behavior is generally using POST exclusively. and using a contrived XML format instead of using ANY headers at all. All requests go to the same motherfucking URL, and all responses are "200 OK" with the same Content-Type. The only hypertext used is for XML schema definitions, all references are as magic strings.
For prominent examples see: SOAP, XML-RPC, and WS-*