WAR is set in the universe of the Warhammer Fantasy tabletop game (not to be confused with Warhammer 40000 - its sci-fi counterpart), and while familiarity with the source material might certainly make the experience a bit richer, it's by no means a requirement. Games Workshop, the developer of the tabletop series, is notoriously strict about licensing out their franchise, and their collaboration with developers Mythic Entertainment is evident: fans of the Warhammer setting will find the game very faithful to the source.
Of all the numerous conflicts and battles in the age of Warhammer, Mythic has opted to focus on three racial pairings for WAR, giving players a choice between six armies to choose from. The humans of the Empire, members of The Order of the Griffon, defend their lands against the army of Chaos (also humans, but... evil humans) from the north - the Raven Host. Greenskins (a term for both Orcs and Goblins) of the Bloody Sun Boyz attack the Dwarf army, the Oathbearers. The third pair is the High Elves of the Shining Guard and the Dark Elves of House Uthorin, embroiled in conflict over their ancestral homelands.
Collectively, these six armies are grouped into two factions, or Realms: the Empire, Dwarfs, and High Elves on the side of Order, and Chaos, the Greenskins, and the Dark Elves on the side of Destruction. The concept of two warring "Realms" - in this case, Order and Destruction - is hardly a new one for MMORPGs, and many gamers who were first introduced to the Massive genre by Blizzard will likely dismiss the factions as carbon copies of the Alliance and Horde (and to be fair, it's not a hard mistake to make). However, while many other games feature the conflict between factions as a sidepiece or bit of flavor, the battle between the Realms is entirely central to WAR.
RvR ("Realm vs. Realm") is the absolute foundation of Warhammer, and the thing that sets it apart from the rest of the genre - the exception being Mythic's previous title, Dark Age of Camelot. The war-torn lands of the three racial pairings are each divided into four Tiers, one for every ten levels of the game (1-10, 11-20, 21-30, 31-40), and most of which consist of two zones apiece. Players are thrust into RvR from quite literally the very beginning; both sides of each pairing share not just the same starting zone, but the same progression between the zones.
Every zone in the game can be controlled by either Order or Destruction, represented by a bar in the corner of the screen. Earning victories for your Realm in PvP will push a slider along the bar, and once it passes a particular point, your Realm will take control of the area. There are three ways to accrue points for RvR: Individual contributions, where killing any enemy player will very slightly tip the scales for your side. There are also objective-based, instanced PvP "Scenarios" (similar to Battlegrounds in Warcraft), and claiming victory in a Scenario will further push the bar. Every zone in the game also has a designated RvR area with objectives for players to capture, and controlling these RvR areas will give the slider a hefty push in your faction's favor.