Tor (software)
Tor is used to hide Internet users.  People who run the Tor network run a network of onion routers.  This makes the location of the user secret.  Services can also be hosted from "secret" locations using the Tor "Hidden services" feature. A hidden service is an anonymous website that search engines like Google don't know about, and it uses an Onion address as its URL. Onion addresses  are URLs that looks like 16 random letters and numbers and ends with .onion. People usually use the Tor browser to go to those sites.