Leagues are a feature that allow you to compete against other Angry Birds Friends players from all around the world. You can increase your League rating by placing well in your group, but beware, your rating can also drop in the higher groups.
When you first start playing in a League, you can create yourself a League profile. This will allow you to choose a separate picture and name that is shown to your competitors.
All competitors are divided into groups of 40 players. This means that even though you might be in same group (i.e. Stone) with your friend, you will not necessarily see each other in the same League group.
Examples of Rating Rewards for different groups
Please note that these ratings may be changed in the future.