One of the first things I had to get used to as a Westerner living in China was going back to a cash economy. For almost the last ten years in the US, I've made virtually all of my purchases -- supermarket, meals, gifts, etc., and certainly anything bought online -- by credit card. If I wasn't carrying cash and for some reason needed some, there were usually plenty of ATMs at hand to spit $20 bills at me.
China doesn't do credit cards. At least, "local" places don't. Places that cater to expats and tourists -- hotels, fancy restaurants, upscale bars, Western supermarkets and department stores, etc. -- will usually take credit cards. A slightly larger number of medium-to-large enterprises (Chinese chain supermarkets, stores in malls, for example; I'm not sure about the convenience stores) will take a Chinese-bank debit card. The market where I buy my produce? Not a chance. Street food stalls? Ditto. Even most restaurants only take cash. And forget about buying things online -- my understanding is that domestic plane tickets can now be purchased online, but not international ones, and I would wager that most people still visit or call a travel agent or airline ticketing office anyway (as for convenience, though, they will hand-deliver your ticket). I carry my credit card in case of some emergency (though I have no idea what could occur for which it would actually come in handy), but I haven't used it in months.

There is, however, one form of plastic payment that is very common over here: the transportation card. These are popular in US (and European) cities as well, but on this side of the ocean they've gained added versatility. The worldwide standard is, as in DC and Singapore (top card in the photo), good for the subway and buses, but as far as I know, that's about it.
In Shanghai, the "traffic card," as it's known here (red Year of the Pig card shown, but there are oodles of designs from which to choose), can be used not only for subway and buses, but also for taxis. That's pretty handy. I once exited a DC cab two blocks early despite being sick to my stomach and ready to pass out (food poisoning, thank you!), all because I didn't have enough cash to cross the zone line to get to my apartment. That's also a reputedly no-longer existent zone issue, but (a) this isn't a DC blog, and (b) all I'm saying is that a stored fare would have been very useful that day.
In Hong Kong, the Oyster card (middle) will not only get you a ride on a bus, subway, trolley, light rail, or ferry (though sadly, not cab), but you can also use it at supermarkets, convenience stores, phone booths (do they still have those?), parking meters, parking garages, and fast food joints. Now, I'm not entirely sure, but if the Hong Kong system is like Shanghai's, you could actually check all of the transactions -- a parent's dream come true! "Mom, I need more money on my Oyster card. I had to take more buses this week." "Oh really...? Should I check on that?" "Um...no..."