An interesting article over at 9to5mac sums it up pretty well:

AirDrop allows you to wirelessly and simply transfer files between nearby Macs, running Lion, and actually does not require users to be on the same WiFi network.
You just have to be nearby and your Mac must have recent WiFi hardware. Chances are though, that if your Mac is capable of running 10.7, you have modern WiFi technology.

So how does it work? They go on to state:

To initiate an AirDrop the involved users simply need to open the AirDrop panel, in Finder, on all their Macs, wait for the Macs to popup on the screen, drag the files you want to transfer, and click confirm.

Pretty cool, we think. And it worked fast and flawlessly when we tried it after installing Mac OS X Lion. Have you tested it out yet? If so, is it working as advertised?