What is "Near-Field Communications"?

I'm told that there are commercials on TV touting Near-Field Communications (NFC): you just hold Android phones back-to-back  and they exchange information.

I'd read about it, but never tried it. I woke up a week ago, thought, "I have a Nexus 7 tablet and a Nexus 4 phone. That should work, right?"

I brought up Google Maps on the phone, held it back-to-back to the tablet and, after a little fumbling, the same map appeared on the tablet.  All this before I got out of bed.

What's going on?

NFC, Bluetooth, Wireless, GPS, and the cell phone itself, are all radio communication. Each of these has a little radio receiver/transmitter in the device that transmits info. Different radios have different ranges, transmission rates, and so on.

Wikipedia offers lots of technical details.

Each one helps me slough wires.  Wireless keeps me from needing an ethernet cable. Bluetooth is shorter range, and keeps me from needing wires to your headphone, my mouse, and so on. NFC is very, very short range, but lets me swap information even if I don't have a wireless network with a router available. If I'm in my car, driving, I can hand my phone to my girlfriend and she can suck information off it onto her tablet.

Going one level deeper, the Nexus boxes are actually using Android Beam, which uses NFC but is actually mostly Bluetooth. The two boxes first exchange enough NFC info to set up a Bluetooth connection, and then pass information back-and-forth with Bluetooth, which is harder to set up, but higher bandwidth.