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.