All of the OWL vocabulary is defined on the 'OWL universe', which is a division of part of the RDF universe into three parts, namely OWL individuals, classes, and properties. The class extension of owl:Thing comprises the individuals of the OWL universe. The class extension of owl:Class comprises the classes of the OWL universe. The union of the class extensions of owl:ObjectProperty, owl:DatatypeProperty, owl:AnnotationProperty, and owl:OntologyProperty comprises the properties of the OWL universe.

There are two different styles of using OWL. In the more free-wheeling style, called OWL Full, the three parts of the OWL universe are identified with their RDF counterparts, namely the class extensions of rdfs:Resource, rdfs:Class, and rdf:Property. In OWL Full, as in RDF, elements of the OWL universe can be both an individual and a class, or, in fact, even an individual, a class, and a property. In the more restrictive style, called OWL DL here, the three parts are different from their RDF counterparts and, moreover, pairwise disjoint. The more-restrictive OWL DL style gives up some expressive power in return for decidability of entailment. Both styles of OWL provide entailments that are missing in a naive translation of the DAML+OIL model-theoretic semantics into the RDF semantics.