BattleTag

BattleTag is a player-chosen nickname that identifies the player on all of Battle.net, including the websites, community forums, Blizzard Games, and more with only one BattleTag that can be associated with a Blizzard account. The BattleTag could also be changed for $10, while the first time changing is free of charge. The BattleTag doesn't follow any unique naming policy aside of the unique identifier that is applied to every BattleTag automatically.

A BattleTag is required to play Diablo III and friends in World of Warcraft can use their BattleTags to communicate across games, realms, and factions, just like Real ID, but do not have to expose their email address to do so. All the grouping and features provided with Real ID works with the BattleTag, minus the real-life name sharing.