The Alterac Valley loading screen
AV starts at level 51, and that bracket goes to 60. Then 61-70.
The goal in AV is to reduce the opposing faction's reinforcement count to 0. The most direct way to do this is to kill the enemy general, which will immediately drain all reinforcement points from the enemy and end the round.
Alterac Valley Map
Other activites which reduce reinforcement points include killing enemy players, capturing towers/bunkers, and killing captains. Reinforcements can be regained by capturing mines. The two generals are raid-level bosses that can be found in their fortresses at the two ends of the map — Vanndar Stormpike ("Vann") for the Alliance north, Drek'Thar ("Drek") for the Horde south.
An overview of Alterac Valley from above.