Battlegrounds are instanced areas used for player versus player combat. Each classic battleground (Warsong Gulch, Arathi Basin and Alterac Valley) has two red instance entrance portals: one for the Alliance, and one for the Horde. Crossing these portals was originally one of the ways to enter the queue for their respective battlegrounds. Now, to enter the queue for any battleground, the player must seek out a Battlemaster, which can be found at the portal or in any capital city, or do so through the Player vs. Player tab. Once inside the instance, the two teams will engage in large-scale PvP, with various objectives determining the winner. Players will earn honor points making PvP rewards available.

If a character goes AFK, he/she will leave the battleground and gain the Deserter debuff. This prevents entry to another battleground for a 15 minute duration. You can report an inactive player by right clicking the player's portrait and selecting "Report AFK". When enough reports are registered, a 60 second debuff will begin to count down. Once the timer is up a new debuff will appear that will prevent the player from gaining any honor or marks while it is on. This debuff can be negated as soon as the player engages in PvP combat. This system is often abused, since a player can have good reasons for not participating in PVP, for example when he is defending.

The two initial Battleground areas, Alterac Valley and Warsong Gulch, went live June 7, 2005. Arathi Basin was added to the list in Patch 1.7 on September 13, 2005. A fourth battleground, named Eye of the Storm, was released with the Burning Crusade. A fifth battleground, the Strand of the Ancients, was added in Wrath of the Lich King. The sixth battleground, the Isle of Conquest, was added in Patch 3.2 on August 4, 2009.

Battleground - WoWWiki - Wikia