Darknet is an overlay network that can only be accessed with specific software, configurations, or authorization, often using non-standard communications protocols and ports.
Two typical darknet types are friend-to-friend networks (usually used for file sharing with a peer-to-peer connection) and privacy networks such as Tor.
Darknet is often used to buy and sell stolen identity data that enable identity fraud.
Ref.: http://www.theinfolist.com/html/en/darknet.html