Web rings are collections of web sites organized around a similar theme.
Each site agrees to incorporate the web ring's code into their site. This code typically provides links to allow your visitors to go to the next site in the web ring, the previous site, to a random site, and to the web ring's central site. The central site lists all of the sites on the web ring. There are literally thousands upon thousands of such web rings. Because there are dozens of sites which organize web rings, there may be dozens of web rings for any given subject.