A web ring is a collection of sites with something in common.
It can be anything that ties the sites together in some way.
Linking sites with a common thread or interest helps people find more sites that interest them directly, resulting in more traffic and more interest to the sites in the Ring.
To make this work, all of the Sites in a particular Ring agree to display a small set of links on their site that let people "travel" forward or backward along the ring, visiting each of the sites in order.
This small set of links is referred to as a "navbar", and it's usually shown as a small, framed rectangle with links for moving forward, backward, and randomly within the ring.
Borrowed from http://webringo.com/