OWL is a family of ontology languages that allows for complex statements about classes and properties. Somewhat simplified, one can say that the standard dialect/language of OWL, known as OWL2 DL, is an extension of RDFS, and can make all statements one can make in RDFS.

OWL2 DL allows statements involving complex class and property expressions. A complex class expression can use intersection, union, and complement of classes. Furthermore, classes can be expressed via a combination of a limited form of quantifiers together with a property and a class to make classes of the form “All people owning a car” or “All people only having female siblings”.

OWL2 DL is designed to maximize expressiveness, while stil being decidable (i.e. resoning always terminates). OWL2 DL does however have a very high complexity for reasoning (2NEXPTIME-complete) due to its high expressiveness. Thererfore, less expressive fragments of OWL2 DL have been made that addresses different use-cases.

OWL is based on a family of logical languages called description logics, and each OWL language corresponds to one description logic.

A sKWRL encoding of (some of) the OWL semantics can be found here.

Guides and tutorials

• IN3060-lectures on OWL (Screencasts available here)
  • OWL basics
  • More OWL
  • OWL: Loose ends
• A Description Logic Primer
• OWL2 Primer
• Wikipedia page on OWL