Tags - Social Classification and Folksonomy
Observing web navigation systems there is a conceptual shift from traditional forms of classification using fixed taxonomies to distributed classification schemes using flexible taxonomies to monitor.
In each of these social systems like
Flickr,
Furl,
Del.icio.us,
Technorati,
GenieLab people classify, bookmark and share different content types like pictures, bookmarks, web pages, blogs and music knowledge with tags, and then the most popular tags float to the top.
When a large number of users all describe the same piece of information with different words the result is that the most used expressions are highest rated and can be designed with individual font sizes, colors and weights for individual attention.
To me folksonomy looks like a very successful navigation system, especially when combined with groups and subgroups.