Ever wondered about those little exhaust pipes protruding from the tails of many jets and turboprops?
An "APU" (Auxiliary Power Unit) is a small turbine engine installed to provide supplementary power. Often found in the tails of larger jets and turboprops, APUs serve several useful purposes.
APU generators provide auxiliary electrical power for running aircraft systems on the ground when the main engines aren't running and no ground electrical power is available. Applications include powering environmental systems for pre-cooling or preheating the cabin, and providing power for crew functions such as preflight, cabin cleanup, and galley (kitchen) operation. Many aircraft APUs can also be operated in flight, providing backup power for the main engine generators.
On larger aircraft, APUs also generate auxiliary "bleed air", referring to pneumatic pressure drawn from the engine's compressor section. That's because large jet engines like those on airliners must be started using pneumatic power. Unless a ground pneumatic source is available, the only way to start large turbine engines is from an operating APU (unless another engine is already running, of course). To accomplish this, the small APU engine is first started using an electric motor (often doing double duty as the generator). Once up and running, APU bleed air is routed to pneumatic starters on the plane's main engines. Those, in turn, spin up the engine compressors for starting.