Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.

In practice, friction, and turbulence cause:

1. non-isentropic compression: for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.
2. non-isentropic expansion: although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work.
3. pressure losses in the air intake, combustor and exhaust: reduces the expansion available to provide useful work.

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Airbreathing jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets, whereas those that generate most of their thrust from the action of a ducted fan are often called turbofans or (rarely) fanjets.
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Auxiliary power units (APUs) are small gas turbines designed for auxiliary power of larger machines, such as those inside an aircraft. They supply compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power. These are not to be confused with the auxiliary propulsion units, also abbreviated APUs, aboard the gas-turbine-powered Oliver Hazard Perry-class guided-missile frigates. The Perrys' APUs are large electric motors that provide maneuvering help in close waters, or emergency backup if the gas turbines are not working.
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