A guild war is a massive PK event between two guilds. First an offer is made by one guild to another, then, if the second guild accepts the offer a war will commence. War generally requires teamwork, strength, and endurance. Large amounts of coordination are also required to win the war.
Usually at the start of the war, one guild will assume control of the battleground. The second guild will then exit the safe area and try to kill all members of the opposing guild. The guild with the most kills at the end of the 30 minute time span wins the war.
Note that during the war, wrath will drop and is able to be gathered mid-battle. Wrath can easily change the tide of battle and minimize the effects of level differences.