Wired has a good & easy read on tagging (saw this on Micro Persuasion)
Wired News: Folksonomies Tap People Power
These days, a growing number of sites whose content is user-created
rely on tagging systems, also known as folksonomies, for the added
value Butterfield is talking about. Flickr and the social-bookmarking
site Delicious, along with Furl, are generally considered folksonomy trailblazers, but now sites like MetaFilter and the blog index Technorati have jumped on board, and more are expected to follow.