Controlled vocabularies, hierarchies, microformats, RDF. Nico Adams pointed me to this excellent video: It’s a really nifty piece of work, which goes into the differences between thesauri, controlled vocabularies, and, as such, ontologies, and social tagging systems. Both have their virtues; it is fuzzy logic versus ODEs all over again. Whether one is better than the other only depends on the problem at hand.