Darknet is a computer network with restricted access that is used chiefly for illegal peer-to-peer file sharing. Its an umbrella term describing the portions of the Internet purposefully not open to public view or hidden networks. This is often associated with the encrypted part of the Internet called Tor network where illicit trading takes place such as the former infamous online drug bazaar called Silk Road. It is also considered part of the deep web.
Darknet can be used for different reasons, few are:
Computer crime (cracking, file corruption, etc.)
Protecting dissidents from political reprisal
File sharing (personal files, pornography, confidential files, illegal etc.)
Whistleblowing and news leaks
Purchase or sale of illicit or illegal goods or services
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. The term has in later usage incorporated services such as the dark web, which is an overlay network that can be accessed only with specific software, configurations, or authorization, often using non-standard communication protocols and ports. As of 2015, the term "darknet" is often used interchangeably with the "dark web" due to the quantity of hidden services on Tor's darknet. The term is often inaccurately used interchangeably with the deep web due to Tor's history as a platform that could not be search-indexed.
Information Source: Link 1