An RDF structure is proposed to describe a listing or a product, categorizable by a URI element (or elements) to a Category's RDF file which provides its name, description, and URI references to its container categories and contained categories.
DARPA Agent Markup Language (DAML) and Ontology Inference Layer/Interchange Language (OIL) are key emerging schemas in this area.
DARPA has a list of ontologies that they've commissioned.
RDFStore is a Perl Module that operates against RDF triples.