The Semantic Web is a vision: the idea of having data on the Web defined and linked in such a way that it can be used by machines not just for display purposes, but for automation, integration and reuse of data across various applications."

From the Semantic Web activity statement.

The World Wide Web has become a vastly powerful tool for communication, research and commerce. However, its power is limited by the ability of human users to navigate the disparate sources for the information they require. The Semantic Web vision is to make the Web machine-readable, allowing computers to integrate information from disparate sources to achieve the goals of end users (see Semantic Web Roadmap). By augmenting web pages with descriptions of the content they hold it becomes possible to reason about that content. The potential impact is huge, representing a reinvention of the world's computing infrastructure on at least the scale of the original web. Enough of the technology is available to build limited exploratory and demonstration applications now. If successful, there is no reason why a popular application could not grow with exponential speed.


Source : HP Labs Semantic Web Research

See also http://www.lowas.be/index.php?pagename=TheSemanticWeb