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Battlegrounds are shared between multiple servers where their players can engage in PvP against other realms. Each group of realms is called a Battlegroup. Battlegroups were introduced in Patch 1.12.
In Patch 3.3.0 cross realm instances were introduced, and they are connected in the same groups as the battlegrounds.
With the introduction of World of Warcraft: Cataclysm and Rated battlegrounds, battlegroups no longer apply to battlegrounds. Players from different battlegroups can PvP against each other in both random and rated battlegrounds if they are in the same region (US, EU, etc.).
Though there is not a current official page dedicated to the North American battlegroups as of yet, players can go Rated Battlegrounds Ladder (US) or Rated Battlegrounds Ladder (EU) and use the drop-down menu to select battlegroups for the rated battlegrounds. Hitting 'filter' will then show the realms listed for that battlegroup, regardless of whether there are any players from those realms engaged in the rated battlegrounds.