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<h1>Frequently Asked Questions on W3C's Web Ontology Language (OWL)</h1>

<p><em>Status: this FAQ is no longer maintained. For a new, up-to-date FAQ, see
the <a href="http://www.w3.org/2001/sw/SW-FAQ">Semantic Web FAQ</a>.</em></p>

<h2>Q. What is an ontology?</h2>

<p>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.</p>

<p>More precisely, from the <a
href="http://www.w3.org/TR/2004/REC-webont-req-20040210/">OWL Requirements
Document</a>:</p>

<blockquote>
  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.</blockquote>

<p></p>

<h2>Q. How is OWL different from earlier ontology languages?</h2>

<p>A. OWL is a <em>Web</em> 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.</p>

<p>OWL rectifies this by providing a language which uses the linking provided
by <a href="http://www.w3.org/TR/2004/REC-rdf-primer-20040210/">RDF</a> to add
the following capabilities to ontologies:</p>
<ul>
  <li>Ability to be distributed across many systems</li>
  <li>Scalable to Web needs</li>
  <li>Compatible with Web standards for accessibility and
  internationalization.</li>
  <li>Open and extensible</li>
</ul>

<p></p>

<h2>Q. What can Web Ontologies be used for?</h2>

<p>A. The Web Ontology Working Group identified major use cases of ontologies
on the Web and described these in <a
href="http://www.w3.org/TR/2004/REC-webont-req-20040210/">the Use Cases and
Requirements document</a>. A survey of implemented applications (using earlier
web ontology languages) was made with about 25 actually deployed systems
identified.</p>

<p>The WG categorized these into six main areas, as follows:</p>

<p></p>
<ul>
  <li>Web Portals
    <ul>
      <li>Categorization rules used to enhance search</li>
    </ul>
  </li>
  <li>Multimedia Collections
    <ul>
      <li>Content-based searches for non-text media</li>
    </ul>
  </li>
  <li>Corporate Web Site Management
    <ul>
      <li>Automated Taxonomical Organization of data and documents</li>
      <li>Mapping Between Corporate Sectors (mergers!)</li>
    </ul>
  </li>
  <li>Design Documentation
    <ul>
      <li>Explication of "derived" assemblies (e.g. the wing span of an
        aircraft)</li>
      <li>Explicit Management of Constraints</li>
    </ul>
  </li>
  <li>Intelligent Agents
    <ul>
      <li>Expressing User Preferences and/or Interests</li>
      <li>Content Mapping between Web sites</li>
    </ul>
  </li>
  <li>Web Services and Ubiquitous Computing
    <ul>
      <li>Web Service Discovery and Composition</li>
      <li>Rights Management and Access Control</li>
    </ul>
  </li>
</ul>

<p></p>

<h2>Q. Who is implementing OWL tools and applications?</h2>

<p>A. A large number of organizations have been exploring the use of OWL, with
many tools currently available. The Working Group is maintaining a <a
href="http://www.w3.org/2001/sw/WebOnt/impls">list of implementations and
demonstrations</a>. 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 <a href="http://www.daml.org">DAML+OIL
</a> (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
<a href="http://protege.stanford.edu/plugins/owl/">Protege system</a>, now
provide OWL support.</p>

<p></p>

<h2>Q. Are there OWL ontologies available already?</h2>

<p>A. There are a large number of ontologies available on the Web in OWL. There
is an ontology library at <a href="http://www.daml.org/ontologies/">DAML
ontology library</a>, which contains about 250 examples written in OWL or
DAML+OIL (a <a href="http://www.mindswap.org/2002/owl.html">converter from
DAML+OIL to OWL</a> is available on the web). In addition, several large
ontologies have been released in OWL. These include <a
href="http://www.mindswap.org/2003/CancerOntology/">a cancer ontology in
OWL</a> developed by the US National Cancer Institute's <a
href="http://ncicb.nci.nih.gov/">Center for Bioinformatics,</a> which contains
about 17,000 cancer related terms and their definitions, and an OWL version of
the well-known <a href="http://www.opengalen.org/index.html">GALEN medical
ontology</a>, developed at the University of Manchester.</p>

<p></p>

<h2>Q. What does OWL add that RDF-schema doesn't?</h2>

<p>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 <a
href="http://www.w3.org/TR/2004/REC-owl-guide-20040210/">OWL Guide</a> provides
examples of all of these in the area of describing food and wine.</p>

<p></p>

<h2>Q. What documents are in the OWL document set?</h2>

<p>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 <a
href="http://www.w3.org/TR/2004/REC-webont-req-20040210/">use cases and
requirements </a> that motivated OWL - an <a
href="http://www.w3.org/TR/2004/REC-owl-features-20040210/">overview</a>
document which briefly explains the features of OWL and how they can be used -
a comprehensive <a
href="http://www.w3.org/TR/2004/REC-owl-guide-20040210/">Guide</a> that
provides a walk-through of the features of OWL with many examples of the use of
OWL features - a <a
href="http://www.w3.org/TR/2004/REC-owl-ref-20040210/">reference document</a>
that provides the details of every OWL feature - a <a
href="http://www.w3.org/TR/2004/REC-owl-test-20040210/">test case document</a>,
and <a href="http://www.w3.org/2002/03owlt/">test suite</a>, providing over a
hundred tests that can be used for making sure that OWL implementations are
consistent with the language design - a document presenting <a
href="http://www.w3.org/TR/2004/REC-owl-semantics-20040210/">the semantics of
OWL and details of the mapping from OWL to RDF</a> (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).</p>

<h2>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?</h2>

<p>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 <a
href="http://www.sciam.com/article.cfm?articleID=00048144-10D2-1C70-84A9809EC588EF21">widely
cited article</a> from <em>Scientific American</em>, Berners-Lee, Hendler and
Lassila wrote:</p>

<blockquote>
  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.
</blockquote>

<p>The OWL language is a major step towards developing that potential.</p>

<h2>Q. What does the acronym "OWL" stand for?</h2>

<p>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:</p>

<p>"He could spell his own name WOL, and he could spell Tuesday so that you
knew it wasn't Wednesday..."</p>
<address>
  Jim Hendler, co-chair of the <a href="http://www.w3.org/2001/sw/WebOnt/">W3C
  Web Ontology Working Group</a>, and the <a href="mailto:w3t-pr@w3.org">W3C
  Communications Team</a><br />
  $Date: 2008/05/21 20:54:18 $
</address>

<p></p>

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