A Horde territory is any area in which Alliance players playing on any server will find their PvP flags turned on. Any Alliance player can be attacked by a player of the Horde while in these zones. Unlike contested territories, it does not matter whether you are on a PvP server or not.
All Horde capital cities are considered Horde territory to Alliance players, regardless of server type. On PvP servers, the Horde territory is extended to encompass all Horde starting areas, as well as Northern Barrens, Azshara, Ghostlands (except for the sub-zone Hatchet Hills, which is contested) and Silverpine Forest.
Alliance players will see a notification on their screens when entering Horde territory.