Global Positioning System, or GPS, was a technology that used satellites to pinpoint an exact location of a certain object or person. Used by typing in coordinates or major location values, this could accurately find anything on Earth. It was often used to track suspects and to follow vehicles without risking the target's suspicions and the surveillance's cover.
Jack Bauer had a handheld GPS device during his infiltration of the Gaines compound, that he used to view satellite imagery of the location. He later gave it to Teri to help her and Kim navigate the surrounding terrain, however they soon got lost. ("Day 1: 11:00am-12:00pm", "12:00pm-1:00pm")
During Day 4, there was a GPS transmitter embedded in the case containing the Nuclear football. To try to protect their locations, Jack told Jason Girard and his wife Kelly Girard, to disable the GPS device. These two unfortunate campers, who happened upon the case, were being tracked by Marwan and his men. ("Day 4: 11:00pm-12:00am")