Warhammer Online: Age of Reckoning (WAR for short) is an excellent player-versus-player-centric MMO. The majority of the game can be played either against computer opponents or else against player opponents - the choice is up to you whether you help your fellow realm-mates complete Public Quests for instant loot rewards and experience or else join up with or form a warband and take the fight to enemy players in the RvR (Realm versus Realm) areas where you fight over Battle Objectives and Keeps (castles). Kill enough of your enemies, capture enough objectives, claim enough keeps, and you will "lock" an entire zone (gaming area) for your realm, winning important bonuses for every realm member in that zone.
Though some people choose not to participate in the RvR content, that is essentially what the game revolves around - and the entire end-game experience is RvR-centric. If you do not like playing with other people against other people, this game is not for you. However, if you would like to join up with a couple hundred of your fellows and cut a swatch of destruction to your enemies' capital city and sack it... this game is for you.