Alliances are key for survival in Travian as you can team up with other players to defeat other players or ask for asistance in battle if you are getting attacked. You need to be in an alliance as soon as possible as high ranked players can easily destroy you without loosing much.
To create an alliance you need an embassy at level 3, to join an alliance you only need embassy level 1.
When joining an alliance, many alliances will request that you use skype, or other instant messaging services. This allows alliance members to chat more freely amoungst themselves.
Often alliances will plan and organize attacks, or defenses by using sites such as: attack-planner.com