Web Ontology Language - OWL

OWL is a core component of the Semantic Web, which is a universe of metadata and ontologies expressed in machine-readable format along with software tools that allow the understanding of semantic relations among heterogeneous and distributed resources in the Web.

OWL is based on technologies recommended by the World Wide Web Consortium, such as the extensible Markup Language (XML), Resource Description Framework (RDF), and Uniform Resource Identifier (URI). It combined these technologies to specify how a document can describe terms and the relationships between them.

The URI technology allows a Web user to uniquely reference a page or other resource, enabling capabilities like displaying a page or downloading a file when a user clicks on a link, or naming distinctly every resource in the Web. RDF and OWL uses the URI to link, talk about, complement, use, and extend distributed resources. (MMI has collected many more references on URIs.)

From the OWL Guide

The OWL language provides three increasingly expressive sublanguages designed for use by specific communities of implementers and users.

  • OWL Lite supports those users primarily needing a classification hierarchy and simple constraint features. For example, while OWL Lite supports cardinality constraints, it only permits cardinality values of 0 or 1. It should be simpler to provide tool support for OWL Lite than its more expressive relatives, and provide a quick migration path for thesauri and other taxonomies.
  • OWL DL supports those users who want the maximum expressiveness without losing computational completeness (all entailments are guaranteed to be computed) and decidability (all computations will finish in finite time) of reasoning systems. OWL DL includes all OWL language constructs with restrictions such as type separation (a class can not also be an individual or property, a property can not also be an individual or class). OWL DL is so named due to its correspondence with description logics [Description Logics], a field of research that has studied a particular decidable fragment of first order logic. OWL DL was designed to support the existing Description Logic business segment and has desirable computational properties for reasoning systems.
  • OWL Full is meant for users who want maximum expressiveness and the syntactic freedom of RDF with no computational guarantees. For example, in OWL Full a class can be treated simultaneously as a collection of individuals and as an individual in its own right. Another significant difference from OWL DL is that a owl:DatatypeProperty can be marked as an owl:InverseFunctionalProperty. OWL Full allows an ontology to augment the meaning of the pre-defined (RDF or OWL) vocabulary. It is unlikely that any reasoning software will be able to support every feature of OWL Full.