The iPad app works in 'layers' in the sense that each time you drill down into something (a tweet, a user account, etc.), it shows that on top of what you were doing before, but still shows a slim strip on the left so that you can go back to where you were.

There's no mixing around, and everything's entirely linear. That's different from a windowing system, in which several different things are clamoring for your attention and mixing together.