1st Difference: Vocabulary

Of the differences between RDFS and OWL, the most important is just that OWL provides a far, far larger vocabulary that you can use to say things.

For example, OWL includes all your old friends from RDFS such as rdfs:type, rdfs:domain, and rdfs:subPropertyOf. However, OWL also gives you new and better friends! For example, OWL lets you describe you data in terms of set operations:

Example:Mother owl:unionOf (Example:Parent, Example:Woman)

It lets you define equivalences across databases:

AcmeCompany:JohnSmith owl:sameAs PersonalDatabase:JohnQSmith

It lets you restrict property values:

Example:MyState owl:allValuesFrom (State:NewYork, State:California, …)

We won't cover any of these predicates right now because, in fact, OWL provides so much new, sophisticated vocabulary to use in data modeling and reasoning that gets its own lesson!

2nd Difference: Rigidity

Another major difference is that unlike RDFS, OWL not only tells you how you can use certain vocabulary, it actually tells you how you cannot use it. By contrast, RDFS gives you an anything goes world in which you can add pretty much any triple you want.

For example, in RDFS, anything you feel like can be an instance of rdfs:Class. You might decide to say that Beagle is an rdfs:Class and then say that Fido is an instance of Beagle:

Example: Beagle rdf:Type rdfs:Class

Example:Fido rdf:Type Example: Beagle

Next, you might decide that you would like to say things about beagles, perhaps you want to say that Beagle is an instance of dogs bred in England:

Example:Beagle rdf:Type Example:BreedsBredInEngland

Example: BreedsBredInEngland rdf:Type rdfs:Class

The interesting thing in this example is that Example:Beagle is being used as both a class and an instance. Beagle is a class that Fido is a member of, but Beagle is itself a member of another class: Things Bred in England.

In RDFS, all this is perfectly legal because RDFS doesn't really constrain which statements you can and cannot insert. In OWL, by contrast, or at least in some flavors of OWL, the above statements are actually not legal: you're simply not allowed to say that something can be both a class and an instance.

This is then a second major difference between RDFS and OWL. RDFS enables a free-for-all, anything goes kind of world full of the Wild West, Speak-Easies, and Salvador Dali. The world of OWL imposes a much more rigid structure.