Of the materials found on the WWW, only a tiny part can be freely accessed and found by search engines. The inaccessible part of the web is called the invisible web (or deep web, hidden web). The invisible web is protected by passwords and firewalls and is not meant for public use. The contents of many catalogues and databases cannot be found with search engines, although these materials are meant for public use. From these databases, materials can be retrieved only by using the search engine of the individual database. Therefore, they, too, can be considered a part of the invisible web.
The invisible web includes
materials protected with a password
documents and interactive tools (e.g. dictionaries, reference works) protected with a firewall
materials held in intranets
databases that are accessible through a dialogue window (e.g. some archives, document databases, library catalogues).
To find materials meant for public use, one has to use subject portals and search engines.