An APU, or Auxiliary Power Unit, is a device used in aircraft and some vehicles to provide energy for functions other than propulsion. It generates electricity and supplies air conditioning while the main engines are off, allowing for comfort and functionality during ground operations. In addition to aircraft, APUs are also found in large vehicles like trucks and buses, where they help power systems without running the main engine. This not only saves fuel but also reduces emissions, making APUs an important component in modern transportation for efficiency and environmental considerations.