Verified answer

Every faction has it's own reputation. You access the reputation pain by pressing "U" if I remember correctly. Otherwise, just open the character pane "C" and click the Reputation or Faction button.

All the main races have a reputation you can earn, but those really don't matter much except for discounts at vendors and you must reach the highest level of Exalted if you want a cross-faction mount.

Other factions such as the ones for battlegrounds and the ones in Outlands and Northrend give rewards in gear and items for getting your reputation higher with them. Most of these reputations also offer special mounts at Exalted as well.

See this page for highly detailed information on reputations:

http://www.wowwiki.com/Reputation

Reputations aren't really all to important till you get higher in level, though the sooner you earn the less youll have to to to get it so to speak. For all the main cities the higher you are in reputation with them you get slight discounts in that city ... which also helps with the cost of mounts and if I remember possible even the skill cost for riding. Reputation really had noting of importance till burning crusade came out. The higher you rep the more items you could by from a specific faction. Same goes with Northrend factions.

rep is sum wat help full but not really till lvl cap say 80 if u have wotlk