Want to keep track of all documents you have greater than 500MB in size? All your PDFs? All your OpenType fonts? Pictures taken with your Canon PowerShot within the last two weeks that have less than a specified focal length and greater than a specified exposure time? If you can set up a search for something in the Finder, you can make a smart folder for it too. These dynamic folders—which don’t actually hold anything, but merely list items stored elsewhere—continually update to display the items on your computer that meet your search criteria. Smart folders can save you time when you would otherwise need to rebuild a search from scratch, helping you find, track, and organize files and folders.
via macworld.com