The idea is that you might put a tag on a document that totally doesn't contain the word you associate with the link.  For instance, a page about building Muppets might contain a pic of David Bowie because of that Labyrinth movie.  The page might not contain any reference to him, and the picture might not be named after him, but when I "tag" that page with his name, you'll be able to find that rare pic of him.\n\nI would guess that any one person's tags are pretty well meaningless, but when you aggregate them all together, meaning emerges.