The Global Positioning System (GPS) is a US possessed service that delivers users with vital information such as positioning, navigation and timing (PNT) services. In simple words, it is a system to calculate the position or coordinate and time on the earth's surface. GPS or officially NAVSTAR-GPS was developed and commissioned by the defence department of US in 1970. The constellation consists of 24 satellites out of which at least four satellites are visible at any time. Each of these transmits information about its position and the current time at regular intervals that are received by the GPS receiver, which then studies the distance of each satellite based on the message arrival time. GPS satellites extends services to civilian and military users base.
Further knowledge: https://en.wikipedia.org/wiki/Global_Positioning_System