Overview
A dark net or darknet is an overlay network within the Internet that can only be accessed with specific software, configurations, or authorization, and often uses a unique customized communication protocol. Two typical darknet types are social networks (usually used for file hosting with a peer-to-peer connection), and anonymity proxy networks such as Tor via an anonymized series of connections.
The term "darknet" was popularized by major news outlets to associate with Tor Onion services, when the infamous drug bazaar Silk Road used it, despite the terminology being unofficial. Technology such as Tor, I2P, and Freenet are intended to defend digital rights by providing security, anonymity, or censorship resistance and are used for both illegal and legitimate reasons. Anonymous communication between whistle-blowers, activists, journalists and news organisations is also facilitated by darknets through use of applications such as SecureDrop.
Terminology
The term originally described computers on ARPANET that were hidden, programmed to receive messages but not respond to or acknowledge anything, thus remaining invisible, in the dark.
Since ARPANET, the usage of dark net has expanded to include friend-to-friend networks (usually used for file sharing with a peer-to-peer connection) and privacy networks such as Tor. The reciprocal term for a darknet is a clearnet or the surface web when referring to content indexable by search engines.
From Wikipedia (CC BY-SA 4.0).