GPS stands for Global Positioning System, and is the name given to the satellite-based navigation system owned and operated by the US government. Primarily designed for military use, the system also found uses in civilian navigation.
The system currently consists of a "constellation" of 31 operational satellites.
Since the launch of the US system, China, Russia, the EU, India, and Japan have built their own GPS systems, and these are called BeiDou (consisting of 35 operational satellites), GLONASS (22 satellites), Galileo (24 satellites), NavIC (7 satellites), and QZSS (5 satellites), respectively.
Apple's iPhone and Apple Watch can use the US GPS, GLONASS, Galileo, QZSS, and BeiDou systems.
GPS satellites send signals at different frequencies, called bands. There are several bands that GPS satellites can transmit on, depending on the system.