David Megginson offers a funny but insightful summary of the most important ideas in REST: The Quick Pitch:

The elevator pitch

With REST, every piece of information has its own URL.

If you just do that and nothing else, you’ve got 90%+ of REST’s benefits right off the bat. You can cache, bookmark, index, and link your information into a giant, well, web. It works you’re reading this, after all, aren’t you? Betcha got here by following a link somewhere, not by parsing a WSDL to find what ports and services were available.