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 and criminals to keep their dealings secret.