In a nut shell, we find out what faction reputation items you are entitled to purchase. read more..
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A faction is a group of allies, either racial or ideological. Most in-game factions are described in Warcraft lore, but some were specifically introduced in World of Warcraft. The concept of a faction exists primarily in World of Warcraft. The use of factions in other Warcraft games is more for general grouping purposes and not quantitatively tracked like World of Warcraft.
Most factions are tracked in World of Warcraft by the reputation that a player character has with each faction. Some groups that are not tracked as factions appear in the organization list.
A character's faction has numerous consequences. Certain zones and cities in the world are also friendly to one faction while hostile to the other. The majority of the levelling experience will be determined by the character's faction, with different quests, NPCs and areas for each faction. Faction also determines a substantial amount of play: work to raise their reputation and earn the praise of the heroes of their faction; complete quests to aid their faction's efforts, and frequently fight against the other faction's NPCs; and generally spend the majority of their time in their faction's cities and surrounded by other members of their faction. Although many choose to play characters of each faction, some players develop a strong sense of faction pride, often leading to varying degrees of sparring and rivalry between players who identify with each faction. - source: Wowpedia
View the full list of factions at Wowhead.
Reputation is very similar to experience. Reputation is divided into a number of different levels for which players must earn reputation points to progress through. Higher reputation levels generally require more points than the previous level to progress. The exception is hated, which spans a very large number of reputation points. Reputation points, just like experience points, are gained through quests and killing various mobs. Unlike experience, it is possible to lose reputation points with some factions, either by killing members of the faction or by assisting rival factions. For rival factions, gaining reputation with one will normally decrease reputation with the other, but at a greater rate. For instance, if you kill a Booty Bay Bruiser, you will gain 25 points of reputation with the Bloodsail Buccaneers, but lose 125 reputation with Booty Bay. Currently there are 4 of these rival-faction pairs: Aldor (Draenei) vs. Scryers (Blood Elves), Steamwheedle Cartel (Goblins) vs. Bloodsail Buccaneers (pirates), Ravenholdt (rogues) vs. Syndicate (Thugs), and The Oracles (Gorlocs) vs. Frenzyheart Tribe (Wolvar).
Various quests and faction-specific rewards are available across the levels, but the player must be able to interact with the faction NPCs to gain access to these rewards. - source: Wowpedia