Postal System
Players can send and receive in-game mail through World of Warcraft's postal system. Every inn has a mailbox that players can right click in order to access mailbox functionality. Read more at Basics: Postal System.
Realm Types
Standard Player vs. Environment Server
Players have the ability to attack almost all opposing faction NPCs, including vendors and quest givers. If a player attacks an opposing faction NPC, players of the opposite faction are then able to target and attack that player. Whenever an NPC is attacked, players who share the same faction as that NPC will receive a system message indicating that a specific town is under attack.
Horde vs. Alliance Race War Server
The Horde vs. Alliance race war server will establish a straightforward method for the development team to test and balance the races and classes in a PvP environment. PvP concepts will be gradually introduced on this server. This means that the initial implementation of PvP is not how our final model will work once World of Warcraft is released. This patch includes the fundamental ideas of PvP in order to tune and refine the very foundation of our PvP model.
Read more at Basics: Realm Types.