The Dark Web (also: Darknet) is a closed network with closed websites that cannot be found by conventional search engines. Furthermore, access via common browsers is not possible; the use of the Tor browser is a prerequisite for accessing the content.
Technically, it is a peer-to-peer overlay network whose participants do not establish their connections to each other via clients but do so manually. The aim is to prevent a third party from easily accessing the network or, ideally, to hide the existence of the network.
In the area of online fraud, fraudsters often use the dark web to trade stolen information (such as credit card data, account access, etc.) or to exchange information about security vulnerabilities at individual online shops.