Introduction to Semantic Web
The Semantic Web is a current project under the direction of Tim Berners-Lee of the World Wide Web Consortium to extend the ability of the World Wide Web by developing standards and tools that allow meaning to be added to the content of webpages. The goal of the semantic web is to create a universal medium for the exchange of data by allowing meaning to be given, using tools and tags, to the content within webpages.
Currently the world wide web contains HTML, which is a language that is useful for displaying graphics and text but does not lend any meaning to the content it describes. The semantic web will address this issue by allowing content to be described in XML documents using tools like RDF and OWL which are types of tags. These description tags that lend meaning to the content facilitates automated information gathering and research by computers.
The semantic web comprises the standards and tools of XML, XML Schema, RDF, RDF Schema and OWL. The OWL Web Ontology Language Overview  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 (in the sense of semantic world wide web), for instance :
- 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, XSLT Extensible Stylesheet Language
- SVG (Scalable Vector Graphic)
- the concept of metadata.
- 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
- Semantic Web Overview
- OWL Web Ontology Language Overview
- W3C Semantic Web initiative
- Tim Berners-Lee's 1998 roadmap paper
- Semantic Web Community Portal
- The Rule Markup Initiative
- International Semantic Web Conference