Piles are a touted feature of future Apple interfaces (Apple own the patent). Here’s a mock-up of how Piles might work:
They are kind of like super spring-loaded folders, without the folder. Advantages are: you can tell just by looking at a Pile what’s in it, and have easy one-click access to all files contained in it. Piles can be various sizes, and you can see at a glance how big one Pile is compared to another.
More on Bruce Tognazzini’s site.