The USGS Water Science School
The generators (big, aren't they?) in this picture are the key to getting electricity from falling water (hydroelectric power). These are very large electric motors containing magnets and wires, quite similar to any small motor, such as in an electric train set. Beneath these generators a metal shaft connected to a propeller is being turned by falling water.
As the propeller is turned by the water, the shaft turns, which then turns the components of the generator to produce electric power. Most electric power is produced in a similar way -- generators don't really care what is used to turn the shaft. Coal-fired power plants burn coal to heat water to produce steam, which turns the shaft; nuclear power plants use nuclear fuel to heat the water into steam.
Hydroelectric power water use Generators in a power plant