Cogeneration (also combined heat and power, CHP) is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat.
Conventional power plants emit the heat created as a byproduct of electricity generation into the environment through cooling towers, as flue gas, or by other means. CHP or a bottoming cycle captures the byproduct heat for domestic or industrial heating purposes, either very close to the plant, or —especially in Scandinavia and eastern Europe—for distribution through pipes to heat local housing.
In the United States, Con Edison produces 30 billion pounds of steam each year through its seven cogeneration plants (which boil water to 1,000?F/538?C) before pumping it to 100,000 buildings in Manhattan—the biggest commercial steam system in the world.
Byproduct heat at moderate temperatures (212-356?F/100-180?C) can also be used in absorption chillers for cooling. A plant producing electricity, heat and cold is sometimes called trigeneration or more generally: polygeneration plant.
Cogeneration is a thermodynamically efficient use of fuel. In separate production of electricity some energy must be rejected as waste heat, but in cogeneration this thermal energy is put to good use.
Reference : http://en.wikipedia.org/wiki/Cogeneration