Alliances are groups of Corporations that work for a united purpose. Think of them as super-Corporations. When someone joins a corporation, they are also joining the Alliance it belongs to (if it belongs to one).
An Alliance has its own private chat area which allows all member corporations to socialize and coordinate groups and attacks.
Each alliance has one planet. When Planets are deployed, it allows members to access Engineering, Terraforming, and the Ship Factory. Planets can also participate in Garrison Engagements as defensive entities.
On Red Server and Blue Server an Alliance can have an unlimited number of member corporations, but only 8 Garrisons can be deployed at once. Green Server only allows 3 corporations and garrisons per alliance.
Alliances cost $1,000,000 Credits on Red & Blue Server to create, and $100,000 Credits on Green Server.
The corporation that creates an alliance is considered to be the Alliance Leader. The new Alliance will be named after the leader corporation (e.g. Atlas corporation makes Atlas alliance). It is possible to transfer leadership to another corporation with Chat Commands. When leadership is moved, the name of the Alliance will be changed.
Any corporation leader that belongs to a corporation that is an Alliance Leader has access to all Alliance Commands. The Alliance and Alliance Planet is theirs to do with as they please.
Alliance Leaders can also invite or kick other corporations from their Alliance.