Nuclear fission is a natural process that occurs with many elements. One or more isotopes of many elements are unstable and they will naturally break apart into smaller pieces. When they break up this process is called nuclear fission. Nuclear because the nucleus of the atom is involved and fission or splitting because the nucleus breaks up into smaller pieces. When fission occurs a small amount of energy is also released as each atom splits. You cannot stop fission from taking place but the rate can be increased. Two examples of fission reactions are nuclear power generation and atom bombs. In nuclear power generation Uranium-235 splits and energy is released. By allowing large numbers of Uranium atoms to split in a controlled manner much energy is released which is used to heat water and turn it into steam. The steam then turns generators to produce electricity.
In an atom bomb large numbers of Uranium atoms split rapidly in an uncontrolled manner and a tremendous amount of energy is released very rapidly.
Nuclear fusion is also a natural process in which the nuclei of small elements are forced together to make larger elements. This process requires very high temperatures and pressures to force the nuclei together. The fusion process is what is believed to fuel the energy release that takes place in the sun and other stars. The only current example of nuclear fusion on earth is when a hydrogen bomb is exploded.