Apple introduced file tagging in OS X Mavericks, letting you assign custom metadata to a file that you can retrieve using Finder. According to our own Brett Terpstra, tagging may be difficult at first, but once we adopt the practice, it may change the way we think about the filesystem of an OS.
With iCloud and sandboxing, Apple is distancing us from the folders that hold our data. In this folder-less existence, Tags will become the glue that holds our data together and lets us organize our files. It'll be an iOS-like experience in which you don't have to worry about where files are being stored. You can find files by their tags and don't have to dig through folders and Finder windows to organize your work.
You can read more about tagging and OS X in his "Mavericks and Tagging" article on his blog.