s&r brought up a most important point about the circle--the chords are sevenths, and as each chord is both the tonic chord in it's own key, and the dominant in the key of the subsequent chord. (G7 resolves to C7 which resolves to F7 which resolves to Bb7 and on around the circle), so they lead into each other, infinitely.

This is the circular part: each dominant chord resolves to a dominant chord, which, because it is unresolved, resolves to the next--as long as you want to go around.

If you want to see how it works, all you have to do is play thru -- C7-F7-Bb7-Eb7-Ab7-Db7-F#7(Gb7)-B7-E7-A7-G7-and C7 around again.

The difference between this and a ladder is that when you get to the top of the ladder, you've got to fall eleven steps to get the first one--