There are two predifined Classes in OWL which are from Description Logic, and are the Top Class and the Bottom Class. In OWL it is owl:Thing and owl:Nothing, the empty Class.
Classes start with a Capital Letter.
Defining an OWL Class is easy. For instance, Denigma is an owl:Class. Thus Denigma is of RDF type owl:Class
owl:Thing (class that contains all individuals) ⊤
owl:[Nothing](/blog/480/) (empty Class) ⊥
Definition of a Class
