Hi Paul!
Okay, chords are built in thirds. If you wanna build a chord where C is the root, you need an E and a G too. You can keep on adding thirds if you want, but three notes is enough for it to be considered a chord.
In any given key, you can build 7 different chords, just by stacking thirds above each of the 7 notes in the scale. If you study harmony, each of these chords is given a roman numeral I II III IV V VI VII, for simplicity.
So, in C major, chord I is C E G. Chord II is D F A. Chord V is G B D. So, in answer to your question, a II-V-I is a progression of chords. You can play a II-V-I in any key, and it will have the same emotional effect. Each chord has an emotional effect (refered to as "function"). For instance, chord I has an effect of resting, of comming home. Chord V has the most tension, and "wants" to go to I. The extent to which I'm simplifying is ridiculous, but what the hell.
Anyway, if you play a II V I using seventh chords (four notes per chord, like C E G B), you will realise it's the staple diet of the jazz language. Well, beebop anyway. It's really boring! I share Scotts boredom with II-V-I.
Now, if you look at the relationship between those three chords, the roots are at a distance of a descending fifth. (D G C) This is a kinda boring relationship, because we have heard it too much. Scott suggests moving arround by thirds instead, like I-III, or I-VI. The magic starts to happen when you start forgetting about being in a particular key, and giving the chords you move to the "wrong" attributes. Tradditionally, chord II (DFA) is minor. Well, make it major if you like [:)] .
So, the concept (which I agree, has been around for about a century and is old news), is to move by thirds, and use whatever kind of chord you like. You are never really in any particular key, and you never get that crappy V-I stark feeling of resolution. Oh, it's also very "Elfman" to move the roots by a tritone (C to F#).
Leon