What is the key operational difference between a fission weapon and a fusion weapon?

The key operational difference between a fission weapon and a fusion weapon is that a fission weapon relies solely on the energy released from the splitting of heavy atomic nuclei (fission), while a fusion weapon uses the energy released from a fission explosion to trigger the fusing together of light atomic nuclei (fusion), resulting in a significantly larger energy yield. A fission weapon, often called an atomic bomb, uses a fissile material such as uranium-235 or plutonium-239. When a critical mass of this material is assembled rapidly, a chain reaction of nuclear fission occurs, releasing a tremendous amount of energy. A fusion weapon, often called a hydrogen bomb or thermonuclear weapon, uses a fission explosion as a trigger to compress and heat isotopes of hydrogen (deuterium and tritium) to temperatures high enough to initiate nuclear fusion. The fusion reaction releases even more energy than the fission reaction, allowing for much larger and more destructive weapons. Therefore, a fission weapon is a single-stage device that uses fission, while a fusion weapon is a two-stage device that uses fission to trigger fusion, resulting in a far greater explosive yield.