I, personally have never used webrings. Webrings are like a group of websites that get together and agree that they will promote and provide links on their site to the other sites in this ring. They share traffic and promote eachother's sites. I think that this is a webring. If I'm wrong, please let me know. Thing is with these rings, you never know what you'll get??? I'm not comfortable promoting sites that I don't know are you? I guess this is what you should ask yourself. If you have a good site, then work hard and make good decisions about your reputation and those you consort with. I'd personally stay away from webrings, linkexchanges and/or anything similar.
It's like a site club, where all the members put a something such as a banner on their site that says "The So-and-so Webring" and links that say "Previous Site" - "Next Site"
Have a look at the WebRing website. It's a concept that's been around since 1994, so it's nothing new.