The deep web (or invisible web or hidden web) is the name given to pages on the World Wide Web that are not part of the surface web that is indexed by common search engines. It consists of pages which are not linked to by other pages, such as Dynamic Web pages. Dynamic Web pages are basically searchable databases that deliver Web pages generated just in response to a query and contain information stored in tables created by programs such as Access, Oracle or SQL databases. The Deep Web also includes sites that require registration or otherwise limit access to their pages, prohibiting search engines from browsing them and creating cached copies.
Non-textual files such as multimedia (image) files, Usenet archives and documents in non-HTML file formats such as PDF and DOC documents used to form a part of deep web, but now are more easily accessible to search engines, especially Google.
The deep web should not be confused with the term dark web or dark internet which refers to machines or network segments not connected to the Internet. While deep web content is accessible to people online but not visible to conventional search engines, dark internet content is not accessible online by either people or search engines.
