"It's hard to imagine that a search engine returning many thousands of hits on a topic you're exploring might not actually have access to all that's out there on the Web. Experts estimate that actually some 80 percent of authoritative content on the Web is invisible to search engines. The Invisible or Deep Web is many times larger, in fact, than the 'surface Web' we have access to via search engines like Google. To better grasp what we mean by the term 'Invisible Web' let's first consider what is visible. Basically, the visible or 'surface' Web refers to any online content that can be found and accessed by a search engine. Search engines send out robot programs or spiders, as they are sometimes called, to crawl the Web looking for pages to add to a search engine database. These pages then get passed to the search engine's indexing program which stores the content in its database. This database in turn is what you're actually searching when you use a Web search engine."
Coffee Break Training - Info Search and Web 2.0 Series No. IS-2009-6
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