Brett Terpstra:
The core concept behind tagging on a computer is that your documents, photos and other files no longer need to have a specific location in the filesystem in order to create relationships (from the user’s perspective, anyway). Apple has long desired to move away from the idea of deeply-nested folders and an obvious filesystem (an idea that, in reality, probably began at NeXT). Spotlight, Saved Searches and other file-locating tools have been in OS X for a long time. In my opinion they’re still undervalued by most Mac users, but Apple is planning to change that.