A gas turbine is a rotary combustion engine that converts the potential energy in gas, plus the kinetic energy of moving air, into a massive amount of energy that turns an output shaft, performing actual work. There are many types of turbine engines, some of which are very familiar to most people, and some of which may be surprising. For example, a windmill is a classic, very simple turbine, and steam engines are turbine engines as well. On the other hand, the US Army's main battle tank, the M1A1 Abrams, is also powered by a gas turbine engine.
Perhaps the use of the gas turbine engine with which most people are familiar is jet propulsion - the gas turbine jet engine. This massive engine illustrates one of the gas turbine engine's great advantages – it has a power-to-weight ratio that's far superior to the other main type of engine available: the reciprocating engine, such as a diesel engine, which harnesses the reciprocating energy of multiple pistons. On the other hand, gas turbine engines are much more expensive to build and operate than reciprocating engines of comparable size, which makes their use cost-prohibitive for use in average consumer applications such as automobiles.
A gas turbine engine consists of three main components, all built around a central shaft: a compressor, a combustion area and the turbine itself, all enclosed so as to be able to build up pressure. Air is drawn into the compressor by fan blades on the shaft, and forced under pressure into the combustion area. Fuel – most commonly kerosene, propane or natural gas – is injected under pressure into the combustion chamber and ignited, significantly increasing the temperature, which in turn increases the pressure in the combustion chamber. This pressurized gas is forced back over multiple fins on the turbine shaft, causing it to rotate and transmit the power to tank wheels or helicopter rotors. Once it's passed over the turbine fins, the spent gas is exhausted, although some gas turbine engines have provisions to capture and reuse some of the heat energy in the exhaust gas.
An aircraft gas turbine engine works a little differently. While the construction remains basically identical, the turbine shaft exists only within the engine itself, because there's no outside destination to which to transmit the power generated by the turbine. Instead, the main job of the turbine shaft is to rotate the compressor fan. The engine's work is performed by the force of the pressurized, heated gas leaving the rear of the engine, providing thrust, which forces the engine housing forward together with the aircraft to which it's attached.