The group of websites who have the details of their IP address hidden intentionally but the pages of the website are visible to people, is known as the dark web.
Dark Web are the websites that can easily be visited by anyone, but their server details cannot be found anywhere on the internet. No one can find out the details about the host of the website.
The dark web is the part of the internet that is completely encrypted for the security purposes. The search engines do not present these websites on the index.
There are two types of websites, one that are easily accessed by the people and the other one is the dark net that are not indexed by the search engines and can be accessed with special software.