As I understand it, rules are prescriptive, mechanics are descriptive. An RPG -role playing game- uses rules. An RPS -role playing system- uses mechanics.
That is to say, in a game the rules tell you how you have to play and lay out what you can't do on moral grounds. While in a system the mechanics tell you what you can't do simply because it is not possible.
Think of the speed of light. That mechanic describes how and why you can't go as fast as light no matter how much power you put into acceleration. You'll get a teeny bit closer, but you'll never reach 1C. It's a simple matter of how the universe works and you're not going to change it.
The same thing with role playing systems. The mechanics of such a system tell you how the system works and you can't change it. Oh, you can change the description -that is, change the mechanics- but all you've really done is change how things work, and that change applies to everything in the system.
That's the clearest I can make it, if you have a better explanation that doesn't contradict what I've said above, drop it into comments.