OWL is built on top of RDF, and while it imposes restrictions on what is allowed to represent, compared to RDF, this is for a good reason. OWL, and especially its subsets OWL-Lite and OWL-DL allows automatic machine reasoning.

In OWL, there are Object properties and Data properties (owl:Property, which is a subclass of rdf:Property). An Object property specifies a relation between Instances, while a Data property specifies a relation between an Instance and a simple data value (string, integer, etc.). Properties can not be used as Classes and vice versa.

Rules of Thumb for OWL DL Ontologies