SKOS is an abbreviation for Simple Knowledge Organisation System. It is an RDF-based ontology for managing taxonomy-related information. As an RDF application, SKOS data can be displayed in xml, N-triples or other rdf-type visual formats. The SKOS ontology describes three classes; Concept (the basic information unit), Concept Scheme (which is used to aggregate Concepts) and Collection (which provides a way to pull together sets of concepts as an supplement to a Concept Scheme.
For more information, look for the W3C SKOS Primer on the web.