Passbook is one of the most useful and, at the same time, underused feature of iOS 7*. From Apple's website...
"Passbook keeps things like airline boarding passes, movie tickets, and gift cards all in one place, letting you scan your iPhone or iPod touch to check in for a flight, get into a movie, redeem a coupon, and more."
Imagine walking into a Starbucks and your loyalty/rewards card automatically appears on your phone's lock screen. Passbook uses your current location to determine the appropriate card to display. You're a simple swipe and scan away from enjoying your favorite beverage. At the airport, suitable boarding pass pops up on your phone's lock screen to scan at security and at the gate.
* Passbook is available in iOS version 6, as well
