Last tour I took this picture in Palm Beach, FL, on one of our very early morning days. I wanted to show you the GPU, or ground power unit. These provide electrical power to the airplane before the engines are started and without running down the aircraft battery. They may be powered by gas, like the one on the left, or by electricity, like the smaller one on the right.
This way, we can turn on all of the radios (so we can get our clearance and plug it into the FMS, flight management system) and especially in FL, turn on the air conditioner! Unfortunately, they are rather loud, and not very environmentally friendly.
Some of the larger aircraft have APUs, or auxiliary power units, which is like an extra engine in the airplane. They are much louder than GPUs, but if I had one, I sure would be using it.
Here's a little more info from Wikipedia: http://en.wikipedia.org/wiki/Auxiliary_power_unit
An aircraft APU is a relatively small, self-contained generator used to start the jet engines, usually with compressed air, and to provide electricity, hydraulic pressure and air conditioning while the aircraft is on the ground. In many aircraft, the APU can also provide electrical power in the air.
A gasoline piston engine APU was first used on the Pemberton-Billing P.B.31 Nighthawk Scout aircraft in 1916. The Boeing 727 in 1963 was the first jetliner to feature a gas turbine APU, allowing it to operate at smaller, regional airports, independent from ground facilities.
Although APUs have been installed in many locations on various military and commercial aircraft, they are usually mounted at the rear of modern jet airliners. The APU exhaust can be seen on most modern airliners as a small pipe exiting at the aircraft tail.
In most cases the APU is powered by a small gas turbine engine that provides compressed air from within or drives an air compressor (load compressor). Recent designs have started to explore the use of the Wankel engine in this role. The Wankel offers power-to-weight ratios better than normal piston engines and better fuel economy than a turbine.
APUs fitted to ETOPS (Extended-range Twin-engine Operations) are a critical safety device, as they supply backup electricity and compressed air in place of the dead engine or failed main engine generator. While some APUs may not be startable while the aircraft is in flight, ETOPS compliant APUs must be flight-startable at up to the aircraft service ceiling. Recent applications have specified starting up to 43,000 ft (≈ 13,000 m) from a complete cold-soak condition. If the APU or its electrical generator is not available, the airplane cannot be released for ETOPS flight and is forced to take a longer route.
APUs are even more critical for space shuttle flight operations. Unlike aircraft APU's, they provide hydraulic pressure, not electrical power. The space shuttle has three redundant APUs, powered by hydrazine fuel. They only function during powered ascent and during re-entry and landing. During powered ascent, the APUs provides hydraulic power for gimballing of shuttle's engines and control surfaces. During landing, they power the control surfaces and brakes. Landing can be accomplished with only one APU working. On STS-9, two of Columbia's APUs caught fire, but the flight still landed successfully.