It has nothing to do with graffiti - just another kind of metadata! Instead of using controlled vocabulary, the web organizes itself from the ground up - users "tag" content in order to search, sort, and find it later.

Tags can be useful in different capacities:

• Tags often describe places. This is popularly referred to as "geotagging." Examples include "Monterey," "Arizona," "France," "Bronx," and so on.

• Tags can describe people, topics, or moods. You might see "depressed," "Sandy," "newbaby," or "history."

• Tags can be based on events. A search for items tagged "il2007" on Flickr will show you participants' photos from the 2007 Internet Librarian conference, or "aasl07" will show you pics from the AASL 2007 conference in Reno. Typically a conference will establish and publicize their chosen tag to coordinate bloggers and publishers as they tag their events.

You will often see tags as "clouds," where the frequency of use is indicated by a larger font size:













Graphic from: http://en.wikipedia.org/wiki/Image:Web_2.0_Map.svg

Others will let you categorize and group them, like del.icio.us' bundle feature:



This bundle, "socialnetworking," includes 7 tags. The number beside each tag refers to the number of links saved under each tag. Web2.0 is a popular tag!



"Schoolstuff" is collapsed and in shadow. If you click on the arrow it will flip down so you can see what's inside.




This bundle called "tools" isn't as dense as "social networking," but it's getting there. Looks like I need to go back into the 22 links tagged "tools" and name the specific tool used... or some other strategy.



But how do I develop my system?

I think that tagging develops in three phases: 1) free-for-all, 2) epiphany, and 3) refinement. At first, you just tag randomly... you're just getting the hang of things. You discover that you can't have spaces (or you create two tags instead of one), and you're observing what terms other people use when they tag. Then, you have a realization! A more refined system is needed so you can use your tags more efficiently. Finally you're ready to edit your tags and make them work for you.