Legions are groups of players who work together as an experience group or as a player killing group. There are no leaders and no one can stop you from joining the legion you want. There are 5 legions to choose from and also 3 professions.

Life - Improved Regeneration
Balance - Improved Signet
Death - Improved Combat Ability
Chaos - Supreme Chaosblast
Order - Greater Quest Point Reward on from Quest Cards

Military - Gain rank through Player Killing for Status
Religious - Gain rank through Sacrificing all junk in rooms
Political - Gain rank by using Bribe

There are 2 additional legions, The Force and Darkside. These are Jedi/Sith only.

It is possible to change your legion using the changelegion command, however, it comes with great costs.