GPS is an acronym for Global Positioning System. The Global Positioning System consists of a set of satellites orbiting the earth, and a set of ground stations that monitor the satellites, determine their positions, compute the parameters of their orbits, and transmit those parameters back to the satellites.
The satellites in turn broadcast their own positions and orbital parameters (the ["ephemeris"]), along with a precise time signal generated by atomic clocks in the satellites. A GPS receiver receives the signals from the satellites and computes the distance to each satellite based on the time it took the signal to reach the receiver. Once the distance has been determined, the GPS receiver uses the ephemeris to compute the location of the satellites, and then uses trilateration to find the position of the receiver, which it then displays as geographic coordinates.