*Information found in databases
*Sites specifically excluded by their owners (these sites have metadata tags that cause search engines to be unable to locate them)
*Sites that are created for single use (i.e. when you enter a search term in Google and receive results, you would not be able to search for that results page at a later time)
Standard search engines like Bing and Google do not have access to the entire world wide web. They only crawl part of the web and therefore, the results that they retrieve are only a portion of the information available.
The invisible web, sometimes called the Deep Web, is the term used to describe all the websites that are not indexed by standard search engines.
These sites are "invisible" to a person conducting a search using a standard search method. Often, the information that is hidden is information that is beneficial to research.