a penstock (also called as steel liners or pressure shafts, depending on     the application), is a conduit for water from the reservoir to flow downward to     the turbine propeller with the force of gravity

  a turbine, with propeller blades attached to a shaft so that the shaft rotates when
    the blades move with the force of falling water. The shaft is attached to the
    generator.

  a generator, that generates electricity when the shaft rotates a series of     magnets (the rotor). The rotor rotates past coils of copper (the stator) wound     outside it. A magnetic field is created between the coil and the magnets, thereby     producing alternating current by moving electrons

  a transformer inside the powerhouse that converts this alternating current into a
    higher voltage current

  power lines that carry the electricity.

  Hydroelectric power plants are built on rivers where there is sufficient drop in   elevation. Hydroelectric power is a renewable source that is non-polluting, has a long   life, lower costs of generation and operation, is more efficient and provides other   benefits such as irrigation, flood control and water supply.