There are four leagues in the game currently: Bronze, Silver, Golden and Platinum. The key difference is the amount of tasks to complete and the reward for winning.
Bronze league allows you to complete 7 tasks + 1 task that can be purchased with diamonds,
Silver league allows you to complete 10+1 tasks,
Golden league - 14+1 tasks and
Platinum - 19 +1.
Guilds that finish the race in the top three are moved to the next league. The only exception is for the Platinum league, you will stay there if you win the race. New leagues will be added in future updates.
Guilds that take three last places will be moved to the previous league (but no further than the bronze league).