This has been answered loads of times on elderscrollsonline.com
On release, there will be part of each of the alliances' territory off-limits (e.g. most of Elsweyr (due to the Knahaten Flu)), but these restrictions will be lifted as they are completed and it makes sense in the game.
Aside from this, characters will be allowed full access to the territory of the faction they belong to, although certain areas will be significantly more challenging than others.
Characters will be allowed to roam other territory once they begin the 50+ content. This is open to characters once they reach level 50. At this point the map will pretty much be open to them.