It's hard to have a meaningful discussion about Apple Pay (iOS' most recent foray into mobile payments) and Google Wallet (Android's three-year-old platform that's had tepid success) without talking about how the systems actually work. And to talk about how those systems work, we have to know how credit card charges work.
We took an iPhone 6 out and found that mobile payments have grown up - a little. It seems like a simple thing, especially in the US - swipe your card, wait a second or two for authorization, walk out of the store with your goods. But the reality is that a complicated system of different companies handles all that transaction information before your receipt ever gets printed.