A blogring is a group of related blogs linked to each other. Multiple blogrings exist for just about every type of web log. They are a place where you can find many blogs related to a particular topic. By joining, you are placing your web site within that ring making it easy for people to find. Rings are also a very easy way to find sites related to a topic you are interested in.
Blogrings can be a way to increase traffic, but it is not guaranteed. Blogrings can be huge and your site may be lost in the masses. However, a well-run blogring that has frequent changes, carnivals, etc. is more likely to bring you traffic than a static (or unchanging) ring that simply lists members.
Some webrings host collaborations called carnivals. The ring owner decides on a topic and invites members of the ring to write on that topic. The owner then provides links to each of those sites that submit links. BlogCarnival.com provides a list of several carnivals ongoing in the blogosphere.