Geolocation

Your position on the earth can be derived in one of the two ways. The first being the GPS satellite system. It's the system used in most cars and, lately, most smart phones. The GPS satellites constantly bounces signals off the earth and there's a special receiver in the GPS hardware that decodes that signal and uses it to determine longitude and latitude. The GPS hardware then returns the coordinates to any software needing them.

The second way is a little bit more complex. All over the world, people have implemented Wi-Fi networks and cell phone tower networks. Most of them are stationary and installed in houses, businesses, cities, and roadsides across the country. In the same way that Google Maps has cars traveling the country taking photos of street views, there are geolocation companies that travel the country and take a census of the available Wi-Fi networks and their relative signal strength and the geolocation of the Wi-Fi measuring device. All of the cell phone towers in the US are already geocoded and their longitude and latitude locations are known. Software can then, with an amazing degree of accuracy determine your location from the Wi-Fi and cell networks and relative strengths available to your desktop, laptop computer, or cell phone without any assistance from GPS-specific hardware.