To be able to participate in the league or tournaments you need a login for the aLobby. Your aLobby login gives you access to complete league-listing, tournaments and your personal game data.
You can log in here with your aLobby-Login.
The League System of the Settlers 3 Community is a competition in which all those who play over the aLobby Settler 3 can fight for the best placement within different rankings. If you wanne participate, simply register under lobby.siedler3.net and have fun :-)
All informations about the League are available in our Wiki.
If you have more question, ask them in our Forum.