Thanks for the pointer. I read the Wikipedia page, but I found the explanation of the general concept too abstract. The definition of a RESTful web service further down the page is much clearer.

However, does it really make any practical difference whether the API makes full use of all the HTTP methods (GET, PUT, POST, DELETE)? Sure, it may be a bit of an abuse of the protocol to encode everything as a GET, but since you usually don't want any caching when using such an API, does it really matter?