Nuclear power is the use of nuclear reactions that release nuclear energy to generate heat, which most frequently is then used in steam turbines to produce electricity in a nuclear power plant.
Nuclear reactors produce just under 20% of the electricity in the USA. There are over 400 power reactors in the world (about 100 of these are in the USA). They produce base-load electricity 24/7 without emitting pollutants (including CO2) into the atmosphere. They do, however, create radioactive nuclear waste which must be stored carefully.