One of the most underused features in Mac OS X is the use of “Smart Folders”. I’m not saying that no one uses them, and you might use them yourself, but from observation of other Mac users, I have found that not many users know about them or what they are.
So…what are they? In a nutshell, a Smart Folder contains items in the folder based on search criteria that you set in the preferences of the folder instead of grouping the contained items by location, like a regular folder. This makes it very easy to better organize your Mac. Say, for example, that you have a lot of word documents or keynote presentations spread out in different folders on your Mac but you want to get them all in one location without physically moving them on the hard drive, then making a smart folder is your solution. Or say that your music isn’t very well organized and you want to get all of your Rage Against the Machine music in one spot, then a smart folder is your solution.
It is very simple to make one. Just open the finder and click File, New Smart Folder. And from there you can set the properties of the folder very easily (by kind, keywords, size, etc.)
Here is the description of a Smart Folder straight from the OS X website…
Smart Folders contain documents grouped together based on search criteria that you define instead of their physical location. Computer file systems are rigid: a file can only be in one folder inside a hierarchy of other folders on your disk. Smart Folders fundamentally change the way you organize your stuff because now a file can literally be in two folders (or many) at once without duplicating or moving the underlying file.
Like Smart Playlists in iTunes, Smart Folders automatically update in real time when you add or remove documents from your Mac. They can contain documents from anywhere in your home directory and external drives, and creating one is as easy as saving a search in the Finder.