A gas turbine is a turbine that extracts energy from a flow of gas, typically air. Intake air is heated by burning after being mixed with some fuel source, then directed out an expanding nozzle, flowing past the turbine on the way.
The most familiar form of gas turbine is the jet engine. Jet engines typically use the power extracted from the turbine to drive the compressor and fuel systems. However another common use is not so obvious. Gas turbines running directly on compressed fuel are used throughout the power generation industry. This is particularly efficient because the heat from the gas turbine can be used to drive a conventional steam turbine in a process known as a combined cycle. This can give efficiencies of 60% of power (electricity) from primary energy.
The use of gas turbines is popular in the electric power industry because gas turbines require relatively little capital investment and can be purchased to generate small amounts of power. They can also be turned on and off within minutes and as a result are useful in satisfying peak demand conditions.