The Semantic Web reference article from the English Wikipedia on 24-Jul-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Semantic Web

The Semantic Web is a project underway that intends to create a universal medium for the exchange of information by giving meaning, in a manner understandable by machines, to the content of documents on the web. Currently under the direction of its creator, Tim Berners-Lee of the World Wide Web Consortium, the Semantic Web extends the ability of the World Wide Web through the use of standards, markup languages and related processing tools.

Table of contents

1 Relationship to the World Wide Web 2 Components of the Semantic Web 3 See also 4 References 5 External links

Relationship to the World Wide Web

Currently, the World Wide Web is based primarily on documents written in HTML, a language that is useful for describing, with an emphasis on visual presentation, a body of structured text interspersed with multimedia objects such as images and interactive forms. HTML does not offer the ability to describe hardly any of the text's content in terms other than in how chunks of text relate to the document as a whole.

For example, with HTML and a rendering tool having appropriate functionality, one can create a catalog of items for sale. In the HTML for the catalog, one can make simple, document-level assertions such as "the document's title is 'Widget Superstore'". But there is no capability within the HTML itself to unambiguously assert that, say, item number X586172 is an Acme Gizmo with retail price of €200, or that it is a type of consumer good. Rather, HTML can only say that the span of text "X586172" is something that should be positioned near the span of text "Acme Gizmo" and "€200", etc. There is no way to say "this is a catalog" or even to establish that "Acme Gizmo" is a kind of title and that "€200" is a price. There is also no way to express the notion that these pieces of information are bound together under relatively intangible categories represented in this particular document by the English phrase "consumer good".

The Semantic Web addresses this issue by making use of the descriptive technologies RDF and OWL, and the data-centric, customizable markup language XML. These technologies are combined in order to provide descriptions that supplement or replace the content of documents on the web. Thus, content may manifest as descriptive data stored in web-accessible databases, or as markup within documents (particularly, in XHTML interspersed with XML, or, more often, purely in XML, with layout/rendering cues stored separately). The machine-readable descriptions allow content managers to add meaning to the content, thereby facilitating automated information gathering and research by computers.

Components of the Semantic Web

The Semantic Web is comprised of the standards and tools of XML, XML Schema, RDF, RDF Schema and OWL. The [OWL Web Ontology Language Overview http://www.w3.org/TR/owl-features/] describes the function and relationship of each of these components of the Semantic Web:

• XML provides a surface syntax for structured documents, but imposes no semantic constraints on the meaning of these documents.
• XML Schema is a language for restricting the structure of XML documents.
• RDF is a datamodel for objects ("resources") and relations between them, provides a simple semantics for this datamodel, and these datamodels can be represented in an XML syntax.
• RDF Schema is a vocabulary for describing properties and classes of RDF resources, with a semantics for generalization-hierarchies of such properties and classes.
• OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. "exactly one"), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.

The usability and usefulness of the Web and its interconnected resources will be enhanced through:

• documents 'marked up' with semantic information (an extension of the <meta> tags used in today's Web pages to supply information for Web search engines using web crawlers). This could be machine-readable information about the human-readable content of the document (such as the creator, title, description, etc., of the document) or it could be purely metadata representing a set of facts (such as resources and services elsewhere in the site).
  (Note that anything that can be identified with a Uniform Resource Identifier (URI) can be described, so the semantic web can reason about people, places, ideas, cats etc.)
• common metadata vocabularies (ontologies) and maps between vocabularies that allow document creators to know how to mark up their documents so that agents can use the information in the supplied metadata (so that Author in the sense of 'the Author of the page' won't be confused with Author in the sense of a book that is the subject of a book review).
• automated agents to perform tasks for users of the Semantic Web using this metadata
• web-based services (often with agents of their own) to supply information specifically to agents (for example, a Trust service that an agent could ask if some online store has a history of poor service or spamming).

The primary facilitators of this technology are URIs (which identify resources) along with XML and namespaces. These, together with a bit of logic, form RDF, which can be used to say anything about anything. As well as RDF, many other technologies such as Topic Maps and pre-web AI technologies are likely to contribute to the Semantic Web.

All current web technologies are likely to have a role in the Semantic Web, including but not limited to the following:

• DOM The Document Object Model, which provides a set of standard interfaces for accessing XML and HTML document components.
• XPath, XLink, XPointer
• XInclude XML fragment XML query language XHTML
• XML Schema, RDF (Resource Description Framework)
• XSL and XSLT, the Extensible Stylesheet Language
• SVG (Scalable Vector Graphics)
• SMIL
• SOAP
• DTD
• the concept of metadata.

Another aspect of the Semantic Web is Friend-Of-A-Friend, FOAF, which allows you to describe yourself in terms of RDF.

See also

• W3C
• Topic maps
• Dublin Core
• WordNet
• DAML+OIL
• OWL
• Cyc
• Knowledge representation, using a semantic network of concepts
• Knowledge technologies
• Web syndication - RSS and Atom/Echo
• Logic programming
• Inference engine

References

• Michael C. Daconta, Leo J. Obrst, Kevin T. Smith: The Semantic Web: A Guide to the Future of XML, Web Services, and Knowledge Management, John Wiley & Sons, ISBN 0-471-43257-1
• Dieter Fensel, Wolfgang Wahlster, Henry Lieberman, James Hendler: Spinning the Semantic Web: Bringing the World Wide Web to Its Full Potential, MIT Press, ISBN 0-262-06232-1
• Vladimir Geroimenko, Chaomei Chen: Visualizing the Semantic Web, Springer Verlag, ISBN 1-85233-576-9
• John Davies, Dieter Fensel, Frank van Harmelen: Towards the Semantic Web: Ontology-Driven Knowledge Management, John Wiley & Sons, ISBN 0-470-84867-7

External links

• Libby Miller and Simon Price: The Semantic Web is Your Friend, FreePint 27th May 2004 No.160
• Semantic Web Overview
• OWL Web Ontology Language Overview
• ZDNet Article
• W3C Semantic Web initiative
• Tim Berners-Lee's 1998 roadmap paper
• Scientific American article on The Semantic Web, May 2001
• Semantic Web Community Portal
• The Rule Markup Initiative
• International Semantic Web Conference
• Friend-of-a-Friend-o-matic

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