what is the difference between owl:Class and owl:Thing?

Question

I know that owl:Class is a subclass of rdfs:class and that all OWL classes are members of owl:Class. I also know that owl:Thing is at the top of the class hierarchy. So what's the difference between owl:Class and owl:Thing? Isn't owl:Class at the top of the class hierarchy?

Answer 1

The most basic concepts in a domain should correspond to classes that are the roots of various taxonomic trees. Every individual in the OWL world is a member of the class owl:Thing. Thus each user-defined class is implicitly a subclass of owl:Thing. Domain specific root classes are defined by simply declaring a named class. OWL also defines the empty class, owl:Nothing.

Every class or object is a 'Thing', if you know java, it's like to say every thing is an object, even java.lang.Class inherit from it

Answer 2

In practice: owl:Class is the URI used to identify the concept of a class in OWL. It's kind of meta information, it's one of the entity type you can use to create your ontology and represent the domain knowledge (there are also owl:ObjectProperty or owl:NamedIndividual types for instance).

owl:Thing is about what your are trying to represent and model. It's the concept that seats in the top of all other concepts and it has to be present on the top by definition.