The W3C's Semantic Web project has been described in many ways over the last few years: an extension of the current web in which information is given well-defined meaning, a place where machines can analyze all the data on the Web, even a Web in which machine reasoning will be ubiquitous and devastatingly powerful. The problem with descriptions this general, however, is that they don't answer the obvious question: What is the Semantic Web good for? The simple answer is this: The Semantic Web is a machine for creating syllogisms. A syllogism is a form of logic, first described by Aristotle, where "...certain things being stated, something other than what is stated follows of necessity from their being so." [Organon]
Not sure I like the logic on his vampire example as the language is missing some simple subtlety in it’s definition.