Introduced In: 10.4 "Tiger (2005)
Virtual folders, or Smart Folders as Apple has called them, weren't introduced in Mac OS X, but it's the OS that brought them to the mainstream. Chances are you've never even created a Smart Folder. Most people have no idea what they are or how they work. Here's a quick run-down.
To create a regular folder, you make a new folder and manually add whatever it is you want to the folder. Simple. A smart folder works by gathering files based on a set of criteria set by you. Say you've downloaded a number of songs over the past week, but they're scattered all over your computer. You can create a smart folder that brings them all together. Even better, the folder will keep adding files that match your criteria. We told you they were smart.