a Round Robin is a type of group quilt. Each person in the group starts with a center (usually) and some times extra fabric and a list of preferences. Then its mailed to the next person in the group (or handed off if a local group). The next person puts on the next 'round'. There is usually a time limit before it needs to be sent to the next person for the next 'round'. Each robin will have its own rules. Some are very planned with each round having its own rules (ie: one border/round pieced, the next applique, the next use a technique that is new to you, etc) Some are very loose and the quilter that is working on that round gets to pick what he/she wants. You never know what will come back!
If you are interested, you can check out the rules of some of the robins to see what they have set up. Hope this helps!