A gas turbine in the broader sense is an internal combustion engine consisting of the gas turbine in the narrower sense (expander) with an upstream compressor and an intermediate combustion chamber. The operating principle is based on the circular process (Joule process) devised by James Prescott Joule: This compresses air via the blading of one or more compressor stages, then mixes this with a gaseous or liquid fuel in the combustion chamber, ignites and burns. The air is also used for cooling. This produces a hot gas (mixture of combustion gas and air), which expands in the downstream turbine section, converting thermal energy into mechanical energy and initially driving the compressor. The remaining portion is used to drive a generator, a propeller, a rotor, a compressor or a pump. In a jet engine, on the other hand, the thermal energy accelerates the hot gas flow, which generates the thrust. In total, over 100,000 large gas turbines are in use worldwide.
The gas turbine is a subclass of the thermal fluid energy machine and thermal turbomachine
Sources:
Wikipedia http://de.wikipedia.org/wiki/Gasturbine