Deep web comprises of   uncatalogued sub-pages of   domains, encrypted   networks, password-     protected data, private   databases etc. The content   in this invisible web cannot   be indexed by the search   engines and so it is generally hard to keep a track of it. Although there is no accurate estimate, but deep web is considered to be 400 to 500 times bigger than the surface web which is a part of World Wide Web that is available to general public and can be indexed by search engines.

Web, in general, can be compared to an iceberg where surface web resembles the visible tip and the huge part hidden below the surface forms the deep web.