A ground receiver uses the signals from at least 3 satellites to determine its location in latitude and longitude. That coordinate can then be used to generate a map. You then know where you are. The US military created the system and uses it to keep an eye on where its own forces are in relation to the other guys'. They've made the system available for civilian use as well.
Hikers can get handheld receivers that track their location as they trek.
And some new cars come with built-in receivers that help their drivers navigate on the go. The FCC uses GPS coordinates to track the location of all broadcast transmitters and translators in the United States. We fed GPS coordinates for IdahoPTV's transmitters and translators into a GIS program, ArcInfo, which then used those coordinates to determine a geographic location for each broadcast tower. As anyone who has ever tried to view or download Hollywood-produced movie clips over the internet knows all too well, high quality is a bandwidth hog. Unfortunately, this high volume of information won't fit into the bandwidth currently allocated for analog TV signals.