from Wikipedia - A darknet (or dark net) is an overlay network that can only be accessed with specific software, configurations, or authorization, often using non-standard communications protocols and ports. Use of darknets is motivated by hiding the content or even the existence of data and communication from competing business or government interests. The most widespread darknets are governmental and corporate intranets, the use of which is a standard security practice nowadays,[1] friend-to-friend[2] networks (usually used for file sharing with a peer-to-peer connection)[3] and privacy networks such as Tor.

It also contains a lot of info about people, illegal sites, and etc. It is said that the "surface web" is only 30% of the entire web.