What is a compass?
A compass is a navigational instrument that indicates the direction to the magnetic poles. It's one of the sensors present in most Android devices.
- calibration: a compass sometimes might need some calibration. Related questions are tagged compass+calibration, see e.g. How does compass calibration work?
- gps: GPS is also used for navigation, often in conjunction with a compass.
- navigation: in most cases, that's what a compass is used for.
- sensors: as said, a compass is a sensor. Your Android device features other sensors, too.