I attended Melbourne wordcamp and caught a couple of presentations. Christine Davis the producer of the ultimate tag warrior did a presentation titles ‘Tags, Categories, Taxonomies, Folksonomies, Oh My!’. Tags where a hot topic of discussion with James Farmer pointing out that wordpress had been slow to include tags as part of the WP CMS until very recently. A key point of discussion was the difference between categories and tags. Apparently, there is a plug-in that convert categories to tags and back the other way on the premise there is no difference. Christine mentioned machine tags introduced by Flickr. From wikipedia:

They comprise three parts, a namespace, a predicate and a value.

Another reference machinetags.org There is also a connection here she suggested with Microformats. Categories where seen as being hierarchal grouping posts of similarity in a group and are more structured, used for organising content. Tags on the other hand more free form used to “determine the way things differ”. A comment in the audience suggested categories are chapters and tags the index. Davis’ response to which to uses suggested using both. Yahoo provide sites/services that recommend what tags to use like zonetag for example.