Basically it is clear from the image that how Hydropower Plant works.
But,I am going to explain it...
Hydropower Plants capture the energy of falling water to generate electricity. A Turbine converts the kinetic energy of falling water to mechanical energy.
The turbine is further connected with generator, so the generator converts the mechanical energy from the turbine into electrical energy.
This generated electricity is then transferred to the transformers and from these transformers it gets transferred to the main power lines.
Hydroplants range in size from " Micro-Hydros " that power only a few homes to giant dams like Hoover Dam that provide electricity to millions of people.