After you reach level 5 or higher, you may choose to join a different guild if you desire. A guild is a group of players of the same occupation, such as smugglers or full-time members of the Alliance. If you choose Alliance or Imperial as your guild, you will also choose a subguild, such as starfighter pilot or ground forces. Since your guild is your occupation, your choice of guild will affect your skill maximum values. Each guild except Neutral has its own set of skills that it can train you well in, and a set which it can only train poorly.
Guilds have headquarters where members can meet, train, talk, and trade equipment.
Joining a guild will also give you access to a guild-member only commline and a new title. Guilds are still in a building stage, so many of the features you might want won't be around for a while longer. To join a guild, you must be Neutral, at least level 5, and find the headquarters of the guild. Usually other guild members will help you if you ask. You must pay 1000 credits, and if your skills are above the levels allowed by the guild, you will lose those skill points.