By gaining reputation with the Stormpike Guard or Frostwolf Clan, players become eligible for rare and epic equipment rewards, as well some consumable items. Items from the Stormpike/Frostwolf factions are identical in most respects, though some have minor differences.
Information on Alterac Valley can also be found at the Official Site.
Because Alterac Valley is not level-banded, unlike Arathi Basin or Warsong Gulch reputation rewards there is only one version of each Alterac Valley reward item. Also unlike AB rep rewards, most AV rep rewards are not class-limited, though they do have level requirements.
Because the rewards for high reputation with these factions are generally useful, and because reputation can be obtained quickly and easily in AV, grinding rep in AV is a popular activity for casual PvPers.