A RESTful Drupal. But what does that mean?

Yesterday, Dries published an important post about the future of Drupal and how it will become a RESTful one. But what is REST anyway? (How I explained REST to my wife) In essence, the fundamentals of the web as we know are already build using a resource-oriented architecture like REST. But it's lacking some aspects, and so is Drupal, which will know get fixed in D8.