A steam engine is an external combustion heat engine that makes use of the heat energy that exists in steam, converting it to mechanical work. Steam engines were used as the prime mover in pumps, locomotives, steam ships, traction engines, steam lorries and other road vehicles, and were essential to the Industrial Revolution. Steam turbines, technically a type of steam engine, are still widely used for generating electricity, but older types have been almost entirely replaced by internal combustion engines and electric motors.

Steam engine in action (animation)

A steam engine requires a boiler to heat water into steam. The expansion or contraction of steam exerts force upon a piston or turbine blade, whose motion can be harnessed for the work of turning wheels or driving other machinery. One of the advantages of the steam engine is that any heatfire fueled by wood, coal or oil or the heat energy generated in a nuclear reactor.

source can be used to raise steam in the boiler