There are constellations of satellites up there beaming positional signals to earth. A GPS unit receives signals from these satellites and uses them to track your position on the earth in three dimensions. GPS stands for Global Positioning System and is the name of the original US satellite constellation.
As well as the US’s GPS, the Russian GLONASS constellation is currently active. The European Union is now deploying the Galileo constellation, which should give more precise positioning than GPS, to ensure that you won’t get lost on your ride if the Americans and Russians turn their systems off. There’s also a Chinese system being deployed called BeiDou/Compass.
Some cycling GPS units just receive signals from the GPS constellation, others use both GPS and GLONASS which should improve positional accuracy and speed of fixing location.
A cycling GPS will have a top screen to display data. Functions of the unit are controlled by buttons on its top or side. More sophisticated units may also have a touchscreen.
The GPS unit attaches to your bike via a supplied mount, usually by inserting it into the mount and giving it a quarter turn. The GPS can easily be removed from the mount for security when you leave your bike.
GPS mounts may be fixed to your bike’s stem or bars with rubber O-rings or via zip ties. Forward mounts are increasingly popular. These replace the supplied mount and position the GPS unit ahead of the bars to improve readability and aerodynamics. They also free up bar space.
A GPS will have a rechargeable battery and a USB port or WiFi functionality to allow you to download data from the unit to a computer. A USB cable is the usual means of recharging the unit too.
The GPS will be resistant to water and dust or dirt entry, so that it can be used in the rain and in dusty conditions.