Also called (mostly in 'the Old Days') power-assisted steering, or simply assisted steering. Power steering is a mechanism present in most modern automobiles that makes it easier and smoother for the driver to turn the car, particularly at low speeds. Power steering systems use a hydraulic mechanism (filled with power steering fluid, naturally) to increase the effect produced when the driver turns the steering wheel. Power steering relies on a power steering pump to produce the hydraulic pressure, meaning that power steering capabilities can be lost if power to the pump is lost (usually due to a problem with a serpentine belt or other power belt). As this would be catastrophic if the car relied on the hydraulic mechanism alone for turning power, power steering equipped cars have fail-safe systems that enable them to operate without power steering if they are required to do so. Nonetheless, a sudden loss of power steering can make controlling a car quite difficult.
In order to maintain the integrity of the power steering system, it's necessary to keep proper levels of power steering fluid in the reservoir connected to the power steering pump. The most accurate fluid levels will be received when the engine is warm, but the dipstick connected to the reservoir may have separate hot and cold level indicators. Power steering fluid leaks can usually be spotted by checking for fluid collecting around the pump or hoses, or by checking for fluid leaking from the steering gear. Most manufactures specify the type of fluid to be used in the power steering system, and different types should not be mixed. A few car models use automatic transmission fluid (ATF), but the two are not necessarily interchangeable. The power steering fluid should not need to be changed outright, unless the pump or steering rack has to be replaced.
Problems with the power steering system are usually heralded by significant added difficulty in turning the car, particularly at low speeds. In some (especially GM) front-wheel drive models, power steering may be sluggish right after the car is started, and then drive normally. This is due to fluctuations in the power steering system caused by temperature-related changes in hydraulic pressure, and may indicate that the steering rack needs to be replaced.
Yahoo autos - http://www.autos.yahoo.com/
AAA - http://csaa.com
Dictionary of Automotive Terms - http://www.motorera.com/dictionary/car-dicp.htm