Xbox One will have its own server for Elder Scrolls Online. Gamers will have the same world to interact with, but will only be playing with other Xbox One users, not the PC or PS4 crowds. The reasons for this choice could be any number of things, ranging from contractual agreements on either Bethesda’s or Microsoft’s side to technical complications involved in rendering on Xbox One as opposed to the infinite variability of PC configurations.
As a gamer, I would hope this is being used as an opportunity, instead of just a limitation. Obviously it limits the available audience for Xbox One gamers to play with, but it also implies certain capabilities Microsoft could implement to offer unique functionality and support to their version of the game if they are so inclined.