"The first level above RDF required for the Semantic Web is an ontology language [an ORL] that can formally describe the meaning of terminology used in Web documents. If machines are expected to perform useful reasoning tasks on these documents, the language must go beyond the basic semantics of RDF Schema (RDF-S).

"OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. 'exactly one'), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes."