Found an exellent discussion about faceted classification – basically the same metadata-based classification we try to implement in Drupal:
Innovation in Classification:
Most folks, when thinking about organizing objects or information, immediately think of a hierarchical, or taxonomic, organization; a top-down structure, where you start with a number of broad categories that get ever more detailed, until you arrive at the object. /…/
Faceted classification, on the other hand, is a bottom-up scheme. Here, each object is tagged with a certain set of attributes and values (these are the facets), and the organization of these objects emerges from this classification, and how a user chooses to access them. Toys, for example, lend themselves to a faceted classification, with the facets being things like, “Suitable Age,” “Price,” “Subject Type,” “Brand,” and even “Character”
No more words, gonna dig deep into it and check all those valuable links they provide.
Another pointer about subject: Next Generation Web Search: Setting Our Sites