Alliances are a group of people who can protect, guide and help you throughout the game. The structure is so far

Overlord - In charge of the alliance
Lord - Second in command to Overlord, can kick/add people, demote/promote people as well as change stance with other alliances
Leader - Third in command Can kick/add people as well as change stance with other alliances
Vassal - Foot soldier

You can join by visiting the alliance page and choosing one to your liking. Or if you want, you can even create your own.

Alliances are ranked according to their power, the power is the total amount of power each member brings to the guild. Alliance members are able to view other members power rating and how long they have been in the allliance. It is also possible to send resources and reinforcements to other member cities. Alliance members can talk to each other by clicking the Alliance tab on the talk box and typing messages to each other although it is becoming more common for an alliance to have their own group on Facebook for doing this and chat attached to the group due to it being quick, not lagging as much and due to the fact that all the alliance can chat to each other whether or not they are on Dragons of Atlantis at the time. Really good alliances will be the most helpfull and friendly, where the teamates rush to help each other whenever they can.