Power producing engines come in two main forms Turboprop and turboshaft.
a. Turboprop Engines. Turboprop engines extract most of the energy from the gas stream and convert it into rotational energy to drive a propeller. The engines are either single or twin shaft and may be direct drive where the LP or main shaft drive the propeller through a gearbox, or they may have a separate power turbine to drive the propeller. Turboprop engines differ from high by-pass turbofans in that the propeller does not have an intake to slow and prepare the air before passing through it. The propeller therefore has to meet the demands of airspeed etc. Examples of turboprops are the Dart, PW125 and Tyne engines.
b. Turboshaft Engines. These engines are used in helicopters. They share many of the attributes of turboprop engines, but are usually smaller. They do not have propeller control systems built into the engine and usually do not have many accessories attached such as generators etc. as these are driven by the main rotor gearbox. Modern turboshaft and turbo prop engines run at constant speed which tends to prolong the life of the engine and also means that they are more efficient as the engine can run at its optimum speed all the time.
There are other types of engine such as ram jets, pulse jets, turbo-ram jet and turbo - rockets, but none of these are used commercially if at all.