Assisted take off
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Any system for helping aircraft into the air (as opposed to strictly under its own power), is known as assisted take off. The reason it might be needed is due to the aircraft's weight exceeding the normal maximum take off weight, insufficient power, or the available runway length may be insufficient, or a combination of all three factors. Assisted take off is also required for gliders, which do not have an engine and are unable to take off by themselves.
Catapults
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A well-known type of assisted take off is the steam catapult, found on aircraft carriers. In this system, a piston, known as a shuttle, is propelled down a long cylinder under steam pressure. The aircraft is attached to the shuttle using a tow bar mounted to the nose landing gear (an older system used a steel cable; the forward ramps on older carrier bows were used to catch these cables), and is flung off the deck at about 15 knots above mimimum flying speed, achieved by the catapult in a 4 second run.
The United States is replacing carrier steam catapults with linear induction motors. The system is called the Electromagnetic Aircraft Launch System (EMALS). An electromagnetic wave travelling through the motor propels the armature along its length, pulling the plane with it. With this system, it will be possible to match launch power and aircraft weight more closely than with the steam system, causing less wear on the aircraft.
The catapult approach is also used for towing gliders into the air. This can be accomplished using an elastic bungee arrangement, or more commonly using a cable wound onto a winch, powered by a large diesel engine. The bungee approach is rarely used for man-carrying gliders, as the acceleration is uncontrolled and can yield very high G-forces. It is commonly used to launch model gliders however. Manned gliders are commonly launched simply by towing them aloft behind a powered aircraft.
JATO and RATO
A system for assisted take off known as JATO for 'Jet-assisted take off' (and the similar RATO, 'Rocket-assisted take off') was popular during the 1950s, when heavy bombers started to require two or more miles of runways to take off fully laden. This was exacerbated by the relatively low powers available from jet engines at the time - for example the B-52 Stratofortress requires 8 turbojet engines to yield the required performance, and still required RATO for very heavy payloads. A proposed update of the B-52 replaces these with half the number of much more powerful engines. In the JATO and RATO system, additional engines are mounted on the airframe which are used only during take off. After that the engines are either jettisoned, or else just add to the parasitic weight and drag of the aircraft.
"Jet Donkeys"
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