From the Q&A sticky "For a player on a budget"
http://www.guildwarsguru.com/forum/s...php?t=10102502
Alliance Battles are exclusive to Factions. The two tribes in Factions, the Kurzicks and the Luxons, each have an area for players to group together into teams of four. Each Alliance Battle randomly groups three of these teams from each side into a cooperative group of twelve, who work together to defeat the opposing group by obtaining more kills and controlling more shrines in the huge arena.
Factions is much shorter, with only 13 missions, two of which are mutually exclusive in the storyline. The game may seem longer than it is due to the questing format of Factions. While Prophecies is more straightforward in that it follows a pattern of “Do mission A, follow quest B to arrive at mission C”, Factions tends to send players back and forth between towns before allowing a player to move on to the next mission. As a result, a player will see some areas of the game multiple times while attempting to proceed in the story. Factions contains only one real grind—but it’s a big grind indeed. At one point mid-game, players must accrue 10,000 unspent factions points for their chosen tribe, the Kurzicks or the Luxons, before moving on in the story. These 10,000 points take quite some time to gain through quests and Alliance Battles—but factions points are also account-based. The prepared player will take multiple characters to that point in the game, accrue the 10,000 points, and gain access for all characters to the next stage in the story before spending the faction points. Apart from this one grind, replaying Factions on new characters is a relatively quick affair—a fact appreciated by some players, tired of Prophecies’ drawn out story, while others are unsatisfied by the shortness.