Electricity has been generated for at least 200 years from various sources of potential energy. The first power plants were run on wood, while today we rely mainly on oil, natural gas, coal, hydroelectric and nuclear power and a small amount from solar power and wind generators.
Most electricity is generated from a heat source, using steam as an intermediate energy carrier. The steam turns turbines that are coupled to electricity generators. The steam can be produced using many of the above mentioned sources of electricity, hydroelectric being a notable exception, in which water flows directly through the turbines powering the generators.
Nuclear power plants often use a primary and secondary steam circuit to add an additional layer of protection between the location of the nuclear fuel and the generator room.
Some small installations (usually burning natural gas) combine the generation of electricity and heat (often for heating homes or industrial processes). These combined power plants have the best energy convertion ratio after hydroelectric plants.