James Robertson at Column Two points to a post at Personal InfoCloud Explaining and Showing Broad and Narrow Folksonomies:
I have been explaining the broad and narrow folksonomy in e-mail and in comments on others sites, as well as in the media (Wired News). There has still been some confusion, which is very understandable as it is a different concept that goes beyond a simple understanding of tagging. I have put together a couple graphics that should help provide a means to make this distinction some what clearer. The folksonomy is a means for people to tag objects (web pages, photos, videos, podcasts, etc., essentially anything that is internet addressable) using their own vocabulary so that it is easy for them to refind that information again. The folksonomy is most often also social so that others that use the same vocabulary will be able to find the object as well. It is important to note that folksonomies work best when the tags used to describe objects are in the common vocabulary and not what a person perceives others will call it (the tool works like no other for personal information management of information on the web, but is also shared with the world to help others find the information).
There I was just getting my head around folksonomies and now we appear to have a taxonomy of folksonomies. It does make some interesting points about the differences between the affordances of tagging in del.icio.us and flickr.
(Via Column Two.)