As to the question of why tones at the interval of an octave sound both the same and different: imagine each pitch as being a color. For example, perhaps E is red. Now, the E in each octave is of a slightly different intensity--the lowest E on a piano keyboard is a very dark brick red, while the highest E is a very light (almost pinkish) red.

On the question of jazz chords: the chords that are used in jazz are just logical extensions of traditional tertial harmony. They did not originate with jazz, however. They first appeared in the music of French impressionists. Hope some of this helped.

klash: Actually, I guess I should explain more completely. Your comment about the visible spectrum being periodic is actually what I intended. I imagine that if we could see above and below the visible spectrum, we would see the visual equivalent of "octaves." That still doesn't really explain it, I guess, but it's the best thing I can come up with.