Google Wallet isn't really one thing, so much as a bundle things tied together in one package. It's an Android app. It's a way for you to pay for things with your credit or debit cards, using your phone. It's a coupon collector and loyalty card system. It's another way for merchants to let you pay and offer up deals. It hooks into other Google services, like Shopper (which shows you nearby deals) and Google Offers. And Google is planning for it to eventually store everything you'd keep in a wallet.
The core payment technology uses wireless NFC and more specifically, MasterCard's PayPass system, so you'll be able to use it anywhere that's hooked up with PayPass, which is at a lot of retailers already. (more)
FutureWatch: Your phone becomes your electronic ID and Passport.
Probably the same reason we still say "dialing the phone" when the dial is long gone, and "the phone is ringing" when a Lady Gaga singtone is belting out the request to connect.
On the plus side, "Hello, Central!" finally exited the lexicon, and "It's your nickle" rates made a come back.