GPS systems for cars  are probably the first applications you can think of but there are other uses for GPS. Handheld gps systems that make hiking in unfamiliar territory an easier task (you can even get wristwatch sized devices).  Fishermen also use GPS to accurately avoid shallows and log locations of fishing grounds.

Another useful GPS application is GPS fleet tracking which allows companies to keep an eye on the location of their goods as they are shipped around the country.  Tracking devices are also useful for keeping an eye on where your children are at any moment (for instance special GPS devices can be packed into a jacket or backpack).

The most common use for GPS systems is in car navigation where you set your destination (no need to set your start point as the device communicates with satellites to determine your exact location).  The device then either shows your route on screen or even talks to you telling you when an upcoming turn is to be made.  If you make a mistake by taking the wrong route the system will calculate a new route and tell you how to get to your original destination.