The two most important technologies for developing the Semantic Web are eXtensible Markup Language and the Resource Description Framework. XML allows content creators to label information in a meaningful way (i.e., <Dryer><Part>95405 </Part></Dryer>). Programs can make use of these tags in sophisticated ways, but the program has to know what the content creator uses each tag for. In summary, XML allows users to add arbitrary structure to their documents but says nothing about what the structures mean.
RDF expresses the meaning of XML. The W3C developed this new logical language to facilitate interoperability of applications which generate and process machine-understandable representations of data resources on the Web. In RDF, a document makes assertions that particular things have properties (such as "is a brother of", "is the CEO of") with certain values. This structure turns out to be a natural way to describe the majority of data processed by machines. Within this structure, the subject and object are each identified by a Universal Resource Identifier, similar to the concept of a link on a Web page. URLs are the most common type of URI.
Even with the above framework in place, two databases may use different identifiers for what is in fact the same concept. A program that wants to compare or combine information across the two databases has to know that these two terms mean the same thing. The program must have a way to discover such common meanings for whatever databases it encounters.
Ontologies provide the solution to this problem. In philosophy, ontology is a theory about the nature of existence, of what types of things exist; ontology as a discipline studies such theories. Artificial-intelligence and Web researchers have co-opted the term for their own jargon, and for them the term ontology refers to a document or file that formally defines the relations among terms.
The Semantic Web will advance the relational database model and overturn old ways of organizing information, according to Berners-Lee. Rather than listing information in tree structures, it will create a Web based on the relationships of people, places and things as they exist in the real world.
He expressed the belief that Semantic Web technology will advance the information revolution he began with the World Wide Web, changing everything from how users set up their online address books to how they pay their taxes.
There is a lot more to the concept of the Semantic Web than can be realistically included in one article. You'll find an in-depth knowledge base on the subject here.
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