Deep web- the part of the World Wide Web not accessible through conventional search engines. dictionary.com
"Just as Deep Web content can’t be traced by Web crawlers, it can’t be accessed by conventional means. The same Naval research group to develop intelligence-gathering tools created The Onion Router Project, now known by its acronym TOR. Onion routing refers to the process of removing encryption layers from Internet communications, similar to peeling back the layers of an onion. TOR users’ identities and network activities are concealed by this software. TOR, and other software like it, offers an anonymous connection to the Deep Web. It is, in effect, your Deep Web search engine.
But in spite of its back-alley reputation there are plenty of legitimate reasons to use TOR. For one, TOR lets users avoid “traffic analysis” or the monitoring tools used by commercial sites, for one, to determine web users’ location and the network they are connecting through. These businesses can then use this information to adjust pricing, or even what products and services they make available."
Taken from "The Ultimate Guide to the Invisible Web" at Open Education Database