There are a lot of different ways to enjoy and explore ESO, one of the most interesting being PvP in Cyrodiil. Fighting in Cyrodiil revolves around the 3 Alliances and their bid for the Ruby Throne. As a player you will fight members of the other alliances as well as capture keeps and other strategic points around Imperial City. However, there are a lot of things to do except fighting. You can explore the world, looking for sky shards, books and resources; or you can choose to do quests and visit dungeons located within Cyrodiil.
There is one very important thing you should know about Cyrodiil and this is the Campaigns.
What is a Campaign?
Elder Scrolls Online useы Megaserver technology. This allows it to house all players on one server. But, you can imagine the chaos that would arise if all players were in one PvP zone. It would be impossible to play the game.
Campaigns are ZeniMax's answer to this problem. A Campaign consists of players from all 3 alliances fighting in Cyrodiil. The goal of a campaign is to restrict the number of players who can fight in one PvP zone simultaneously therefore making the fighting less chaotic. There are multiple campaigns operating within the game at the same time, each with its own progress.
For example, there can be 30 instances of Cyrodiil and each consists of players from all 3 factions. If there are too many players in several instances the Megaserver will create one more and will place players there to alleviate overpopulation.
Joining a Campaign
Once you create your character the game automatically assigns you to one of many campaigns. You will be assigned according to your Faction and Campaign population. When you reach level 10 you can visit Cyrodiil from the Alliance War interface. This is your home campaign.
You can also participate in other campaigns as a guest player, for instance, it is possible to jump into another campaign if you're invited into it. This won't unassign you from your home campaign, you will just play with your friend for some time. The bad thing about playing in other campaigns is that you won't increase your leaderboard rankings in your home campaign.
Changing your Campaign
There can be situations when you need to change your current campaign. Maybe your friend is assigned to a different one or you just want to play with other people. Elder Scrolls Online allows you to leave your current campaign and join a different one of your choosing, so long as it isn't overpopulated.
Every character can change their campaign once for free. But, any other changes after this will cost you Alliance Points or Gold. This is a failsafe against players jumping from one campaign, to the other, every 5 minutes when they feel they aren't winning and will allow campaigns more stability.
You should also keep in mind that each campaign has its own leaderboard. It is unknown if your achievements will also be transferred to the new campaign, but we doubt it. So keep this in mind if you wish to switch campaigns.
There are also other restrictions in place for those who wish to choose a campaign. For example, you will not be able to join a campaign if you have a character in another faction assigned to it, this is to prevent spying.
Finally, If a player with Emperor status changes his or her home campaign, that player will lose the emperorship. This is to prevent several Emperors being in the same instance of Cyrodiil. That about covers all you need to know about how campaigns work within ESO, all you have to do now is join one and begin the fight for the ruby throne.