Blizzard is implementing a new social feature for their games called BattleTags. BattleTags are a player-chosen nickname that can be used across all Blizzard games, websites, and forums.
"Similar to the Real ID system, BattleTags will ultimately give players on Battle.net a new way to manage public profiles, find and chat with friends they've met while playing, form groups, and stay connected across multiple Blizzard games," said Blizzard.
The big difference between Real ID and BattleTags seems to be the information being displayed. When you give out your Real ID, you're letting people know your e-mail address and real name. Not everyone's cool with giving that information out to say, a World of Warcraft guildmate. BattleTag essentially lets you stay in touch with people without sacrificing your privacy.
Most of the BattleTag features have not been implemented yet. The in-game chat and friends list are live, but only in the Diablo III beta. Even if you're not in this beta, though, you can now claim a BattleTag through the Battle.net Account Management page.
There's no need to rush to grab a BattleTag. These tags aren't unique. When you create the nickname, Blizzard will attach a random 4-digit number to it. Also note that you can only have one BattleTag per Battle.net account, and there's currently no way to change it, so you ought to take your time thinking of a tag.
BattleTags will be integrated into Diablo III at launch. The feature will be incorporated into World of Warcraft and StarCraft II at some point as well but no time table was provided.