Frequently Asked Questions on W3C's Web Ontology Language (OWL)

Status: this FAQ is no longer maintained. For a new, up-to-date FAQ, see the Semantic Web FAQ.

Q. What is an ontology?

A. Although the concept of ontology has been around for a very long time in philosophy, in recent years it has become identified with computers as a machine readable vocabulary that is specified with enough precision to allow differing terms to be precisely related.

More precisely, from the OWL Requirements Document:

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 domain information (a domain is just a specific subject area or area of knowledge, 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.

Q. How is OWL different from earlier ontology languages?

A. OWL is a Web Ontology language. Where earlier languages have been used to develop tools and ontologies for specific user communities (particularly in the sciences and in company-specific e-commerce applications), they were not defined to be compatible with the architecture of the World Wide Web in general, and the Semantic Web in particular.

OWL rectifies this by providing a language which uses the linking provided by RDF to add the following capabilities to ontologies:

Q. What can Web Ontologies be used for?

A. The Web Ontology Working Group identified major use cases of ontologies on the Web and described these in the Use Cases and Requirements document. A survey of implemented applications (using earlier web ontology languages) was made with about 25 actually deployed systems identified.

The WG categorized these into six main areas, as follows:

Q. Who is implementing OWL tools and applications?

A. A large number of organizations have been exploring the use of OWL, with many tools currently available. The Working Group is maintaining a list of implementations and demonstrations. In addition, both the US government (via DARPA and NSF) and the European Union (via the 5th and 6th generation frameworks of the IST program) have invested in web ontology language development. Most of the systems currently using DAML, OIL and DAML+OIL (the predecessor languages that OWL was based on) are now migrating to OWL. In addition, a number of ontology language tools, such as the widely used Protege system, now provide OWL support.

Q. Are there OWL ontologies available already?

A. There are a large number of ontologies available on the Web in OWL. There is an ontology library at DAML ontology library, which contains about 250 examples written in OWL or DAML+OIL (a converter from DAML+OIL to OWL is available on the web). In addition, several large ontologies have been released in OWL. These include a cancer ontology in OWL developed by the US National Cancer Institute's Center for Bioinformatics, which contains about 17,000 cancer related terms and their definitions, and an OWL version of the well-known GALEN medical ontology, developed at the University of Manchester.

Q. What does OWL add that RDF-schema doesn't?

A. Owl extends RDFS to allow for the expression of complex relationships between different RDFS classes and of more precise constraints on specific classes and properties. Example of these include: - the means to limit the properties of classes with respect to number and type, - the means to infer that items with various properties are members of a particular class - the means to determine if all members of a class will have a particular property, or if only some of them might - the means to distinguish one-to-one from many-to-one or one-to-many relationships, allowing the "foreign keys" of a database to be represented in an ontology - the means to express relationships between classes defined in different documents across the web, - the means to construct new classes out of the unions, intersections and complements of other classes, and - the means to constrain range and domain to specific class/property combinations. The OWL Guide provides examples of all of these in the area of describing food and wine.

Q. What documents are in the OWL document set?

A. The Working Group has produced six documents each aimed at different segments of those wishing to learn, use, implement or understand the OWL language. Our documents include - a presentation of the use cases and requirements that motivated OWL - an overview document which briefly explains the features of OWL and how they can be used - a comprehensive Guide that provides a walk-through of the features of OWL with many examples of the use of OWL features - a reference document that provides the details of every OWL feature - 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 - a document presenting the semantics of OWL and details of the mapping from OWL to RDF (This document presents the model theoretical details of every feature of OWL so that those implementing complete OWL reasoners can guarantee algorithmic compliance with all aspects of the language design).

Q. What is new about ontologies on the Semantic Web? How do they differ from expert systems and the other artificial intelligence (AI) technologies promoted in the 1980s?

A. The relation between the Semantic Web, and OWL in particular, to work in AI is somewhat parallel to the relation between the Web and the hypertext community -- based on some of the same motivations, but with a very different architecture that drastically changes the ways in which the technology can be deployed. In a widely cited article from Scientific American, Berners-Lee, Hendler and Lassila wrote:

For the semantic web to function, computers must have access to structured collections of information and sets of inference rules that they can use to conduct automated reasoning. Artificial-intelligence researchers have studied such systems since long before the Web was developed. Knowledge representation, as this technology is often called, is currently in a state comparable to that of hypertext before the advent of the Web: it is clearly a good idea, and some very nice demonstrations exist, but it has not yet changed the world. It contains the seeds of important applications, but to realize its full potential it must be linked into a single global system.

The OWL language is a major step towards developing that potential.

Q. What does the acronym "OWL" stand for?

A. Actually, OWL is not a real acronym. The language started out as the "Web Ontology Language" but the Working Group disliked the acronym "WOL." We decided to call it OWL. The Working Group became more comfortable with this decision when one of the members pointed out the following justification for this decision from the noted ontologist A.A. Milne who, in his influential book "Winnie the Pooh" stated of the wise character OWL:

"He could spell his own name WOL, and he could spell Tuesday so that you knew it wasn't Wednesday..."

Jim Hendler, co-chair of the W3C Web Ontology Working Group, and the W3C Communications Team
$Date: 2008/05/21 20:54:18 $