The Elder Scrolls Online, for those not already aware, will be an MMORPG developed by Bethesda's parent company Zenimax (but not Bethesda themselves). It splits the continent of Tamriel into four giant pieces, three of them will be player factions for you to choose your side and the fourth is Cyrodiil, the central region they are fighting over.
The Daggerfall Covenant is made up of the Orcs, Bretons and Redguard. The Aldmeri Dominion is made up of Altmer, Bosmer and Khajiit. And the Ebonheart Pact is made up of the Nords, Dunmer and Argonians. Though you'll recognize familiar settings like Skyrim, Cyrodiil and Morrowind if you've played recent Elder Scrolls games, they may only be passingly familiar as the game is set over a thousand years in the past.
Current talk is that players will not be able to enter into 'enemy' territories. So if you are a member of the Ebonheart Pact you will have access to Morrowind, Skyrim and Black Marsh and can enter the PvP area of Cyrodiil, but will not be able to enter High Rock, Hammerfell, Summerset, etc. Many other details of how the MMO will be set up have yet to be released.
Zenimax also surprised fans with a beta sign up: Elder Scrolls Beta, and the mad scramble it has created among fans has left their servers working overtime. So if it's not up at the time you are reading this, then try again later.