So what is it?
Nuclear Energy is produced through a process called "nuclear fission". By shooting an extra neutron into a special isotope of an uranium atom, the atom will split, releasing two new atoms, three neutrons, and plenty of energy. These three neutrons go on to hit more atoms, causing a chain reaction throughout the uranium. Meanwhile, the energy produced from these fissions is known as Nuclear Energy.
In a nuclear power plant, the energy released from these reactions is used to power turbines, which in turn transform the energy into electricity that is then sent out to power the world.
The Chernobyl disaster is probably the most well know nuclear accident to date. A wrongly designed reactor core was used inside the plant, and the operators were not properly trained to work the machines. This resulted in a steam explosion and fires that released much of the radioactive material into the air around. 30 people were killed within the next week as a result of radiation poisoning.