I really don't know where to start this post, but let's start with imagining the following scenario/situation/fact:
You have different files on your disk ranging from music to videos to documents to pictures to applications to system files, all these files are arranged in a hirarchial way, for example, you have your My Music folder under My Documents wich is under your name and so on... Under My Music you probably have different folders for different artists inside which you have folders for each album which in turn have your music files, what if you wanted to see all your rock song files in one page? (forget about media players for now).
You also have documents ranging from tutorials you downloaded from the net to your work, and just for the sake of explanation, let's assume that your source code files are documents too (which they are), now, what if you wated to organize your files, you've been working on a prjrct for your school where would you put your source files? Will you put them in the folder in which you keep your school work and assignments or in the flder in which you keep your projects? What about the tutorials you used to learn more about some library you used to create your prject? Where will that go?
You see, it is really hard to organize files in a hirarchy and find them easily, if you want to access one of your songs, say Rise Inside by Killswitch egaged you're gonna have to navigate through C:/Documents and Settings/<your username>/My Documents/My Music/Killswitch Engaged/Alive or Just Breathing/Rise Inside.mp3, I know, I know, you can have shortcuts in windows and links in *nix sytems but that's not an answer, smply because if you click on the My Documents shortcut on the desktop you don't actually open a My Documents that's in your dekstop, it's the same one, think of command line users, they can't even use these shortcuts, links in *nix systems are a bit better but they still can't solve the problem, you can link files but you can't link directories, well, you can create symblic links for directories but what if you dcided to delete the original one? The link is useless now!
Now, lwt's imagine the same situations with a tag file system.
For the first scenario you can simply add tags your music files with generes, years, artist names, etc... So if you want all the rock songs that were released in 1965 all you have to do is to open /rock/1965/ and you should find all the artists, albums, etc... If you want to view all the files that have these tags you can use /rock/1965/*/ where the astirisk means all the tags that are here...
For the second scenario you simply have to tag the files with college, prject and you're good to go, if you want to view your college files it will show there and if you want to view your projects fils it'll show there too, it's the same file, in the same place only accessed using different tags.
For the third scenario, tags do the trick and more, if you want to access your my documents you just have to open /My Documents/ and you have youe My Documents folder open, it's the same one that you can access from somewhere else.
What can I do? Well, I've been wondering about tha myself, don't know if I should merge it with Junia, make an FS for *nix systems based on libfuse (note that there is one already and it's called tagsistant), or just stick to theory.
Vista was supposed to ship with WinFS which was supposed to do something similar to this, but it didn't ship with it.
Tags have been getting more popular these days, take chandler for example, it was supposed to be a revolutionary PIM that gets rid of hirarchies but, it's been years sonce it started and they haven't reached version 1.0 yet.
On the other hand, you can see may websites using tags to arrange their content, from bookmarks to emails.