MacRumors.com

This US Patent Application (appears better in IE than Safari) gives the nitty-gritty details of Apple's implementation of 'Piles' (AKA 'Method and Apparatus for Organizing Information in a Computer System'):

The patent was filed in 1994, but has gained interest with recent rumors that the next major revision of Mac OS X (Panther) will utilize this design.

The function of a pile is, of course, the grouping of documents. While a good description was provided by this AskTog article, it spawned a surprising amount of confusion regarding implementation amongst readers.

For those interested, the Patent description provides more details on the actual implementation and also describes advanced sorting/searching functions, including:

- analysis of the content of a document in a pile
- automatically filing new documents according to criteria (date, content etc...)
- piles looking neat or disheavled per user preferences
- create/sort into subpiles based on criteria (date, content etc...)
- script based sorting of documents