Daniela Barbosa, who is the business development manager for Synaptica at Dow Jones Client Solutions has written a fabulous eBook explaining how to mix taxonomies built by professionals and folksonomies, which let users tag content as they see fit
Modern facet analytical theory contrasts with earlier views of knowledge as an integral whole (which is broken down into smaller and smaller units) in that it deals with individual terms or concepts which are clustered into categories to create a 'bottom-up' map of knowledge.