The conversion of kinetic energy of running water to electrical energy is known as hydroelectric Power.Water is stored in a high lake or reservoir. At s height gravitational potential energy is stored in the water.When water falls from height, the potential energy is changed into Kinetic energy.
Tunnels are made to bring water from the reservoir to a lower place. The kinetic energy of running water turns the turbines which in turn runs the electric generators. By this method electricity is produced.
In this method, coal ,oil, and natural gas are burnt. The remaining of plants and animals buried for millions of year under the earth are changed into fossil fuels.These fuels are in limited quantity .Chemical potential energy stored in fossil fuels,the burning of these fuels gives out heat which is used to generate Stream that turn the turbines to produce electricity.
The source of nuclear is the nucleus of an atom in which energy stored. When the nuclear of a heavy atom is broken, a large amount of energy in the from of heat is released. This process is known as nuclear fission.Uranium-235 or Plutonium is used as fuel in nuclear fission.
This process is done in nuclear reactor which is protected by a concrete wall. The heat obtained by nuclear fission is used to change water to steam,which in turn runs the electric generators.So electricity is produced.
Sunlight is directly transformed to electricity with the help of solar cells. The voltage of one cell is very small but for practical use connect many cells in series to get large voltage .
This method is expensive but it is hope to get it cheaper in future.