Alliances are groups of independent factions who work together as a group towards a single goal. Those goals are things like (and not limited to) trade, mutual defense, world domination.
This Server is designed for both group play and solo play however, solo play is more difficult and having Allies at your side to help in times of crisis is always an advantage.
Joining an alliance is a simple process. Figure out which alliance best suits your play style and is recruiting. Then speak with their leader and request to join the alliance of your choice. Each alliance leader has the power to add new players to their alliance.