clipped from: blog.wired.com 
Google recently announced it will soon begin indexing the so-called "deep" web, those pages hiding behind HTML forms and other inadvertently spider-blocking HTML elements. The move will potentially open up a whole new range of webpages that were previously invisible to the search engine.
The deep Web (or Deepnet, invisible Web or hidden Web) refers to World Wide Web content that is not part of the surface Web indexed by search engines. It is estimated that the deep Web is several orders of magnitude larger than the surface Web.[1]