What are "WOW" battlegroups?

In "World of Warcraft," battlegroups are groups of player subgroups or realms that can battle each other in arena fights. Players who are in different battlegroups cannot fight each other regardless of what region or server they play on.

Since the "World of Warcraft: Cataclysm" expansion pack, battlegroups are no longer enforced as long as players are part of the same server or region. This means that United States players can only fight other players from the United States, European Union players can only fight other players from the European Union, and so on. However, players can fight with or against players from anywhere within their region as long as they are playing the Cataclysm expansion.