Richard Newman has a writeup on his Tag ontology design including N3 notation for the draft ontology itself. The purpose is to create a generic way to associate tags with content with richer context than what is used for instance by Technorati.
One of the advantages of Richard's design is that by using RDF tags can be distributed separately from the content itself - for instance tags occuring in an RSS feed (for instance from Technorati or del.icio.us ) and in your blog could assert that a certain post is tagged with a certain tag in exactly the same way, or third parties could publish collections of tags from their own categorisation, again in the same format.
More over at Richards blog