WNW: Rules? Who Says?

Several people have asked me recently about the rules of grammar and usage. Who makes them? What determines what a rule is? When and why and can we break them?
To answer that, first, we have to back up a bit.
Language evolves; we all know that. Take a look at the opening line of Chaucer's Canterbury Tales in the original, and your eyes will glaze over. (That is English? What the . . . )
My dad has the first paragraph memorized, and when he recites it, it sounds stinkin' cool . . . but nothing...