Bobby Bucks™ is a fan-created slang term for the real money generated by or spent in the Diablo III Real Money Auction House. The term refers to Activision/Blizzard President Bobby Kotick, whose avarice is legendary. Bobby's constant drive to monetize everything in all Activision/Blizzard games is widely assumed to be behind the introduction of this revenue-generating system, though Blizzard has always insisted their motives behind the RMAH were entirely about enhancing the player experience.
Blizzard's official term for Bobby Bucks is, Battle.net Balance.