RDFS or RDF Schema is an extensible knowledge representation language - http://en.wikipedia.org/wiki/RDFS
It sits on top of RDF (ResourceDescriptionFramework) in the SemanticWebLayerCake and encodes information about the meaning of the items used in RDF. This makes it possible to exchange information using e.g. DublinCore to define commonly used terms.
See also PracticalRdf, ExplorersGuide (to the Semantic Web).