A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Sir Charles Parsons in 1884. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 90% of all electricity generation in the United States (1996) is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible expansion process.
For More on Steam Turbines: http://en.wikipedia.org/wiki/Steam_turbine
Steam nozzle can be considered as a passage of varying cross-section by means of which the energy of steam is converted into kinetic energy. The nozzle is so shaped that it will perform this conversion of energy with minimum loss. One may also define a nozzle as an opening through which steam is passed from a region of high pressure to one of lower pressure so as to derive additional velocity. It is chiefly used for producing a large velocity steam jet. In other words, its chief use is to produce a jet of steam for the purpose of driving steam turbines. The function of a nozzle in an impulse turbine is to admit steam to the active or moving parts of the turbine. In a reaction turbine the stationary nozzles admit steam to the moving parts which are also of nozzle shape and guide the steam from them.