I watched a Lynda.com course today by David Gassner on Web Services. (It was well done. I highly recommend it.) Anyway, Gassner states that "REST is an architecture, not a messaging format." I interpret this to mean that REST conventions, unlike SOAP conventions, are far more flexible. In other words there are many permutations of REST and RESTful APIs.
This question reveals how new I am to this topic, but hey, you gotta start somewhere.