Nuclear fission

In case you were absent from physics class that day, here's a visual explanation of the concept behind atomic energy. A neutron is fired at an isotope of uranium, which causes it to split and throw off other neutrons. Each neutron hits another uranium nucleus, which then splits and throws off neutrons, causing a chain reaction. Each time an atom is split, it gives off a tremendous amount of energy in the form of heat. That heat is what creates steam to drive a turbine to make electricity in a generator.