From gbhackers.com
The deep web, invisible web, or hidden web are parts of the World Wide Web whose contents are not indexed by standard search engines for any reason.
The opposite term to the deep web is the surface web. The deep web includes many very common uses such as webmail, online banking but also paid for services with a paywall such as a video on demand, and much more.
This part of the Internet is known as the Deep Web, and it is about 500 times the size of the Web that we know.