WoW lets you choose for your super close friends to either Real ID or Battletag. This lets you talk to the person whenever they're on, regardless of what character they're playing. You can even talk to them if they're playing Starcraft or Diablo. With Real ID you give them your email, which them shows the name you registered your WoW account under. With Battletag, you give them a number which is universal across all your Blizzard games and lets you do the same things, but without giving your name. It's a pretty neat system.