The concept is simple: money as an email attachment. And it's not fundamentally more capable than PayPal, which has let you send money to other people for years. But in online payments, ease counts for a lot — preexisting iTunes accounts are a large part of why the App Store caught on so quickly, for example. Which is why this, which was buried among flashier announcements about Chrome, Glass, Android, Music and Maps, might be the most important thing Google's announced in months.