A steam turbine is a rotary machine usually utilized by thermal power plants which converts the kinetic energy of the movement of high pressure steam into rotary mechanical energy, which can be converted into other forms of energy, such as electrical energy, or the turbine may be used to turn submarine or ship propellers.
One must note that steam (water vapour/water in gas form) is 1600 times more voluminous than liquid water, and steam power plants take advantage of this by boiling liquid water using a variety of heat sources, which causes the water to expand, and then pressure increases inside the boiler, because heat causes expansion, and that causes what is called a phase change from liquid to gas. The area outside of the boiler is a much lower pressure, therefore the steam is going to “try to” exit the nozzles at the end of the pipeline at a high speed and then eventually hit the blades, and that force pushes the blades, turning them.
Steam turbines can be used to turn electricity generators, the propellers of large ships and submarines (particularly military ones) directly, or they may turn electricity generators that supply electricity to electric propulsion motors and other electrical/electronic items.
Nuclear powered water vehicles use nuclear reactors to generate heat which is used to heat a boiler/steam generator which then supplies steam to steam turbines. Steam turbines are also used for waste heat powered electricity generation as well, which is when waste heat from a gas turbine is used to heat a boiler which generates steam and supplies it to the steam turbine(s). The efficiency of such a setup can exceed 60%.
Steam turbines are also used in combined cycle power plants that use waste heat from combustion engines to produce steam, which is of supplied to steam turbines.