Metadata? Thesauri? Taxonomies? Topic Maps! "Information architects have so far applied known and well-tried tools from library science to solve this problem, and now topic maps are sailing up as another potential tool for information architects. This raises the question of how topic maps compare with the traditional solutions, and that is the question this paper attempts to address."
Offers a good introduction to the ideas of taxonomies, ontologies, faceted classification, controlled vocabularies, and of course, topic maps.