You’re it! Folksonomies and tagging
Folksonomy is the practice of collaborative cateforization using freely chosen keywords. As an internet feature it was developed in 2004 with advances in social software. Some examples: del.icio.us, social bookmarking; Flickr, photo sharing; 43 Things, goal sharing; GenieLab, music recommendations/associations. Folksonomy stems from "folk classifications", how average people (non-experts) classify the world around them, which have long been studied in sociology and anthropology. The term is a portmanteau or frankenword (like brunch, smog, or infomercial), mixing folk and taxonomy (taxis=classification, nomos=management), hence people’s classigication management, coined by Thomas Vander Wal (wonderwall 
, an information architext working for the INDUS Corporation in Bethesda, Maryland. As with most things, the fastest way to learn is to do.
To use del.icio.us, type in del.icio.us/tag/(instert subject here) in the url. You’ll get a list of recent tags with that subject, as well as a list of related tags. Click on the tagger’s name to see of what else they’ve tagged. You can also subscribe to tags of interest, by clicking subscribe towards the top. If you have Firefox, here is an extension called Feedview that manages newsfeeds (you can have your tags fed through RSS [Really Simple Syndication]). If you use a different browser, but deep down feel unsatisfied with it, download Firefox.