Hydroelectric power refers to the turning of a turbine or waterwheel caused by the flow of water over it. In modern times this rotating turbine is used to produce electricity which powers all kinds of machinery and equipment, however, hydroelectric power has in fact been utilised by mankind for many centuries.
Today's hydroelectric power plants function by water stored in a dam being forced down by gravity through a large pipe known as a "penstock". As it falls it spins a turbine which is connected to an electric generator, from where the electricity created is transported through wiring to the Grid or to where it is required. The water which has passed over the turbine is released out into a river or ocean. A nice moving diagram of the whole process can be found here.
A hydroelectric power plant has a number of environmental benefits over other forms of power generation, as it will produce no harmful emissions and should not pollute the water providing the machinery parts are well-maintained and are not leaking a harmful substance such as oil or hydraulic fluid.
As well as the benefits for the environment, hydroelectric power generation is much better at adjusting to the peaks and troughs of energy demand at different times of the day and night. Whereas other sources of power such as fossil fuel burning power stations take a long time to start up or shut down, hydroelectric power can be switched on or off relatively quickly. Some plants also have the ability to pump water back up to the reservoir above during off-peak times so that a source of water is then available for use during peak electricity requirements.
Despite the high costs involved in constructing them, with a need for the turbine and dam construction, hydroelectric plants normally have a long life span so will justify the initial cost spent over their existence.
Along with the high initial financial outlay, the drawbacks of hydroelectric power are primarily of an environmental nature. The dams associated with hydroelectric power facilities can be visually intrusive upon the natural landscape, whilst the requirement for a reservoir at the top can mean that houses and wildlife habitat in the area needs to be relocated or destroyed to make way for this large body of water.