Strictly speaking, the GPS actually encompasses the entire network of solar powered satellites, which form part of New Inventions in Electronics. They encircle the globe at about 19 300km. These satellites make two complete rotations every day, and their orbits are arranged so that at anytime, anywhere on Earth, there are always several or more satellites visible in the sky.
These satellites continually transmit messages containing, among other bits of information, the time the message was sent, the precise orbital information and the orbits of all the other GPS satellites. In a nutshell, a GPS receivers job is to locate four or more of these satellites, precisely time their transmitted signals, figure out the distance to each, then, using a simple mathematical principle called trilateration and pinpoint its own location.
A standard GPS receiver will place you on a map at any particular location, but will also trace your path across the map as you move.