The "Deep Web" is everything that Google or other search engines are unable to crawl due to the vastness of the web. Google publicly admits to this phenomenon but understates the vastness of content that is not indexed in their system.
Although Google is an adequate search for most people, sometimes the best information is not in the top results. In fact, the cross-linking part of Google’s algorithm can skew the results and be manipulated by those with a little know-how. To find the best information, a deep web crawler can be employed by a data extraction company or a stand-alone program. The deep web crawler can bypass the manipulation and even read meta-data that is not actually visible on the site. As well, it can find more obscure but relevant sites and all the information is brought back by the deep web crawler and converted into one of many formats such as cvs or txt.
For a comparison of programs able to crawl the deep web and help index it for your personal needs follow this link: theeasybee.com