(Fast summary: According to Tenney, just as we usually divide time into years, months, days, and so on, music can be divided hierarchically. This hierarchies arise in music because of musical differences from moment to moment (and phrase to phrase, section to section, etc.). The shapes of these differences — and the similarities that bind sections together — are how form emerges.)
Today I found a great piece that demonstrates this kind of thinking: Psappha by Iannis Xenakis. The most obvious of the hierarchies is created by the different percussion sounds, but you can also hear groupings emerge due to the different rate of events and the pauses between them.
For the truly nerdy, you can go read my paper and delve more into how this works.
Otherwise, you can just enjoy the cool sounds of an artsy Guitar Hero (ht: Josh Harris).