The Elder Scrolls Online Wiki
The Elder Scrolls: Online is an epic MMO from ZeniMax Online Studios, which is the first Elder Scrolls game to allow players to explore the series' universe with others. Like previous Elder Scrolls games, The Elder Scrolls: Online is set on the world of Nirn. Unlike previous games, it features online, mulitplayer action.
During the Second Era, about 1000 years before the events in Skyrim, Tamriel descended into a four-way war. In an attempt to conquer the continent, the Imperials – natives to the province of Cyrodiil – made a deal with the Daedric prince Molag Bal. At the beginning of the game, players will join one of three factions that not only oppose the Imperials and Molag Bal's undead army, but will also battle one another for the imperial throne. Three different alliances. Three different philosophies. Three different ways to approach the game. Which will you choose?
Bethesda announced a new update with the name One Tamriel at the E3 conference. This update will make it possible for players to explore the game world more freely, without level, alliance or quest restrictions. 
The only place the alliance restriction won't change, is the PVP area.