The “dark web” is a part of the world wide web that requires special software to access. Once inside, web sites and other services can be accessed through a browser in much the same way as the normal web.
However, some sites are effectively “hidden”, in that they have not been indexed by a search engine and can only be accessed if you know the address of the site. Special markets also operate within the dark web called, “darknet markets”, which mainly sell illegal products like drugs and firearms, paid for in the cryptocurrency Bitcoin.
There is even a crowdfunded “Assassination Market”, where users can pay towards having someone assassinated.
Because of the the dark web’s almost total anonymity, it has been the place of choice for groups wanting to stay hidden online from governments and law enforcement agencies. On the one hand there have been whistleblowers using the dark web to communicate with journalists, but more frequently it has been used by paedophile groups, terrorists andcriminals to keep their dealings secret.
There are a number of ways to access the dark web, including the use of Tor, Freenet andI2P. Of these, the most popular is Tor (originally called The Onion Router), partly because it is one of the easiest software packages to use. Tor downloads as a bundle of software that includes a version of Firefox configured specifically to use Tor.
Tor provides secrecy and anonymity by passing messages through a network of connected Tor relays, which are specially configured computers. As the message hops from one node to another, it is encrypted in a way that each relay only knows about the machine that sent the message and the machine it is being sent to.
Rather than conventional web addresses, Tor uses “onion” addresses, which further obsure the content. There are even special versions of search engines like Bing and Duck Duck Go that will return onion addresses for Tor services.
It is a mistake to think that Tor is entirely anonymous. If a web site is accessed, it can still potentially find out information about whoever is accessing the site because of information that is shared, such as usernames and email addresses. Those wanting to stay completely
Source: What Is The Dark Web?