NPC, an abbreviation of Non-Player-Character, describes all characters in the game that are not directly player controlled, including PvE foes and the friendly inhabitants of your realm. In a RvR game such as WAR, NPCs who are friendly towards PCs from one realm may be aggressive towards PCs from another realm.
WAR will have a variety of NPC roles for players to interact with:
Friendly NPCs will be generally linked to one of the playable armies in the game, and it's been hinted that their behavior towards characters will vary depending on which army they belong to. For example, High Elf NPCs will behave differently depending on whether they are dealing with another High Elf or a Dwarf.
It's been stated that NPCs from every army in Warhammer will make an appearance somewhere in the game.