http://en.wikipedia.org/wiki/Wardriving ,
Simply put , people just drive around with a usb wifi dongle , and capture their wifi information (AP, SSID's, Encryption) with something like NetStumbler or inSSIder or kismet, and also capture their gps coordinates.
Its done to find out all the wifi's in your area, with this information , if the wifi encryption is weak, someone can crack it , with something like Fern-Wifi-Cracker and a Usb Dongle that supports Packet Injection , so if your short of cash or need to do something evil , you can use the victims wifi.