With the daily addition of million documents and new users, there is no doubt that the World Wide Web (WWW or Web shortly) is still expanding its global information infrastructure. Thanks to low-cost wireless technology, the Web is no more limited to homes or offices, but it is simply everywhere. The Web is so large and growing so rapidly that the 40 million page "WebBase" repository of Inktomi corresponds to only about 4% of the estimated size of the publicly indexable Web as of January 2000 and there is every reason to believe these numbers will all swell significantly in the next few years. This unrestrainable explosion is not bereft of troubles and drawbacks, especially for inexpert users. Probably the most critical problem is the effectiveness of Web search engines: though the Web is rich in providing numerous services, the primary use of the Internet falls in emails and information retrieval activities. Focusing in this latter, any user has felt the frustrating experience to see as result of a search query overwhelming numbers of pages that satisfy the query but that are irrelevant to the user.