Leagues are groups of guilds that are formed based on the total amount of contributions to guild quests. What league your guild is in and what place it is in within that league defines the size of the reward you will get after completing guild quests.

There are 7 Guild Leagues in the game:
1) Wooden—all guilds start in this league;
2) Bronze;
3) Silver;
4) Gold;
5) Emerald;
6) Ruby;
7) Royal—guilds in this league receive the most valuable reward for completing guild quests.