The World Wide Web Consortium (W3C) has published a suite of six Candidate Recommendation specifications defining the Web Ontology Language (OWL). An emerging ontology standard designed to strengthen the Semantic Web foundations, OWL is "a language for defining structured, Web-based ontologies which enable richer integration and interoperability of data across application boundaries. Early adopters of these standards include bioinformatics and medical communities, corporate enterprise and governments. OWL enables a range of descriptive applications including managing web portals, collections management, content-based searches, enabling intelligent agents, web services and ubiquitous computing. OWL is already being used as an open standard for deploying large scale ontologies on the Web."

The six Candidate Recommendation documents for OWL are written for different audiences, addressing variable needs in understanding and implementing the OWL language. These include: "(1) a presentation of the use cases and requirements that motivated OWL; (2) an overview document which briefly explains the features of OWL and how they can be used; (3) a comprehensive Guide that walks through the features of OWL with many examples of the use of OWL features; (4) a reference document that provides the details of every OWL feature; (5) a test case document, and test suite, providing over a hundred tests that can be used for making sure that OWL implementations are consistent with the language design; (6) a document presenting the semantics of OWL and details of the mapping from OWL to RDF." Public comment on the OWL CR documents is requested by 20-September-2003.

OWL Web Ontology Language Candidate Recommendation: Bibliographic Information

• OWL Web Ontology Language Overview. Edited by Deborah L. McGuinness (Knowledge Systems Laboratory, Stanford University) and Frank van Harmelen (Vrije Universiteit, Amsterdam). W3C Candidate Recommendation 18-August-2003. Latest version URL: http://www.w3.org/TR/owl-features/. "This document is written for readers who want a first impression of the capabilities of OWL. It provides an introduction to OWL by informally describing the features of each of the sublanguages of OWL. Some knowledge of RDF Schema is useful for understanding this document, but not essential. After this document, interested readers may turn to the OWL Guide for a more detailed descriptions and extensive examples on the features of OWL."

• OWL Web Ontology Language Guide. Edited by Michael K. Smith (Electronic Data Systems), Chris Welty (IBM Research), and Deborah L. McGuinness (Stanford University). W3C Candidate Recommendation 18-August-2003. Latest version URL: http://www.w3.org/TR/owl-guide/. "This document demonstrates the use of the OWL language to formalize a domain by defining classes and properties of those classes, to define individuals and assert properties about them, and to reason about these classes and individuals to the degree permitted by the formal semantics of the OWL language. The sections are organized to present an incremental definition of a set of classes, properties and individuals, beginning with the fundamentals and proceeding to more complex language components."

• OWL Web Ontology Language Test Cases. Edited by Jeremy J. Carroll (HP) and Jos De Roo (AGFA). Latest version URL: http://www.w3.org/TR/owl-test/. "This document contains and presents test cases for the Web Ontology Language (OWL) approved by the Web Ontology Working Group. Many of the test cases illustrate the correct usage of the Web Ontology Language (OWL), and the formal meaning of its constructs. Other test cases illustrate the resolution of issues considered by the working group. Conformance for OWL documents and OWL document checkers is specified."

• OWL Web Ontology Language Use Cases and Requirements. Edited by Jeff Heflin (Lehigh University). W3C Candidate Recommendation 18-August-2003. Latest version URL: http://www.w3.org/TR/webont-req/. "This document specifies usage scenarios, goals and requirements for a web ontology language. An ontology formally defines a common set of terms that are used to describe and represent a domain. Ontologies can be used by automated tools to power advanced services such as more accurate Web search, intelligent software agents and knowledge management."

• OWL Web Ontology Language Reference. Edited by Mike Dean and Guus Schreiber. Authors: Sean Bechhofer, Frank van Harmelen, Jim Hendler, Ian Horrocks, Deborah L. McGuinness, Peter F. Patel-Schneider, and Lynn Andrea Stein. W3C Candidate Recommendation 18-August-2003. Latest version URL: http://www.w3.org/TR/owl-ref/. "The Web Ontology Language OWL is a semantic markup language for publishing and sharing ontologies on the World Wide Web. OWL is developed as a vocabulary extension of RDF (the Resource Description Framework) and is derived from the DAML+OIL Web Ontology Language. This document contains a structured informal description of the full set of OWL language constructs and is meant to serve as a reference for OWL users who want to construct OWL ontologies."

• OWL Web Ontology Language Semantics and Abstract Syntax. Edited by Peter F. Patel-Schneider (Bell Labs Research, Lucent Technologies), Patrick Hayes (IHMC, University of West Florida), and Ian Horrocks (Department of Computer Science, University of Manchester). W3C Candidate Recommendation 18-August-2003. Latest version URL: http://www.w3.org/TR/owl-semantics/. "This description of OWL, the Web Ontology Language being designed by the W3C Web Ontology Working Group, contains a high-level abstract syntax for both OWL DL and OWL Lite, sublanguages of OWL. A model-theoretic semantics is given to provide a formal meaning for OWL ontologies written in this abstract syntax. A model-theoretic semantics in the form of an extension to the RDF semantics is also given to provide a formal meaning for OWL ontologies as RDF graphs (OWL Full). A mapping from the abstract syntax to RDF graphs is given and the two model theories are shown to have the same consequences on OWL ontologies that can be written in the abstract syntax."

Web Ontology Language Overview

"The OWL Web Ontology Language is designed for use by applications that need to process the content of information instead of just presenting information to humans. OWL facilitates greater machine interpretability of Web content than that supported by XML, RDF, and RDF Schema (RDF-S) by providing additional vocabulary along with a formal semantics. OWL has three increasingly-expressive sublanguages: OWL Lite, OWL DL, and OWL Full."

"The Semantic Web is a vision for the future of the Web in which information is given explicit meaning, making it easier for machines to automatically process and integrate information available on the Web. The Semantic Web will build on XML's ability to define customized tagging schemes and RDF's flexible approach to representing data. The first level above RDF required for the Semantic Web is an ontology language what can formally describe the meaning of terminology used in Web documents. If machines are expected to perform useful reasoning tasks on these documents, the language must go beyond the basic semantics of RDF Schema. The OWL Use Cases and Requirements document provides more details on ontologies, motivates the need for a Web Ontology Language in terms of six use cases, and formulates design goals, requirements and objectives for OWL."

"The World Wide Web as it is currently constituted resembles a poorly mapped geography. Our insight into the documents and capabilities available are based on keyword searches, abetted by clever use of document connectivity and usage patterns. The sheer mass of this data is unmanageable without powerful tool support. In order to map this terrain more precisely, computational agents require machine-readable descriptions of the content and capabilities of Web accessible resources. These descriptions must be in addition to the human-readable versions of that information. The OWL Web Ontology Language is intended to provide a language that can be used to describe the classes and relations between them that are inherent in Web documents and applications. The OWL Guide document demonstrates the use of the OWL language to: (1) formalize a domain by defining classes and properties of those classes; (2) define individuals and assert properties about them; (3) reason about these classes and individuals to the degree permitted by the formal semantics of the OWL language..."

OWL 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 constraints. For example, while it 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 OWL Lite provides a quick migration path for thesauri and other taxonomies. Owl Lite also has a lower formal complexity than OWL DL, see the section on OWL Lite in the OWL Reference for further details.

• OWL DL supports those users who want the maximum expressiveness while retaining computational completeness (all conclusions are guaranteed to be computed) and decidability (all computations will finish in finite time). OWL DL includes all OWL language constructs, but they can be used only under certain restrictions (for example, while a class may be a subclass of many classes, a class cannot be an instance of another class). OWL DL is so named due to its correspondence with description logics, a field of research that has studied the logics that form the formal foundation of OWL.

• 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. 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 complete reasoning for every feature of OWL Full. [adapted from the Overview and Guide]

From the Announcement

"An ontology defines the terms used to describe and represent an area of knowledge. Ontologies are used by people, databases, and applications that need to share subject-specific (domain) information -- like medicine, tool manufacturing, real estate, automobile repair, financial management, etc. Ontologies include computer-usable definitions of basic concepts in the domain and the relationships among them. They encode knowledge in a domain and also knowledge that spans domains. In this way, they make that knowledge reusable.

The W3C Web Ontology Working Group carries a complement of industrial and academic expertise, lending the depth of research and product implementation experience necessary for building a robust ontology language system. Participants include representatives from Agfa-Gevaert N. V; Daimler Chrysler Research and Technology; DARPA; Defense Information Systems Agency (DISA); EDS; Fujitsu; Forschungszentrum Informatik (FZI); Hewlett Packard Company; Ibrow; IBM; INRIA; Ivis Group; Lucent; University of Maryland; Mondeca; Motorola; National Institute of of Standards and Technology (NIST); Network Inference, Nokia; Philips, University of Southampton; Stanford University; Sun Microsystems; Unicorn Solutions along with invited experts from German Research Center for Artificial Intelligence (DFKI) Gmbh; the Interoperability Technology Association for Information Processing, Japan (INTAP); and the University of West Florida.

OWL brings together research from a number of groups that have been developing languages in which to express ontological expressions on the web. OWL has its origins in two major research efforts: a draft language known as the DARPA Agent Markup Language Ontology notations (DAML-ONT) and Ontology Interface Layer (OIL) developed by European researchers with the support of the European Commission. Since then, an ad hoc group of researchers formed the Joint US/EU committee on Agent Markup Languages and released a new version of this language which merges DAML with the OIL. The documents released today reflect the collaborative work of international researchers with industrial participants working together the World Wide Web Consortium.

W3C's Semantic Web Activity builds on work done in other W3C Activities, such as the XML Activity. Its focus is to develop standard technologies, on top of XML, that support the growth of the Semantic Web. At the foundation, XML provides a set of rules for creating vocabularies that can bring structure to both documents and data on the Web. XML gives clear rules for syntax; XML Schemas then serve as a method for composing XML vocabularies. XML is a powerful, flexible surface syntax for structured documents, but imposes no semantic constraints on the meaning of these documents. RDF (the Resource Description Framework) is a standard a way for simple descriptions to be made. What XML is for syntax, RDF is for semantics -- a clear set of rules for providing simple descriptive information. RDF Schema then provides a way for those descriptions to be combined into a single vocabulary. What's needed next is a way to develop subject- or domain-specific vocabularies. That is the role of an ontology." [adapted from the press release]