Fusion and Fission both produce energy, but they are different processes. Fusion involves combining two or more atoms to form a heavier atom, and occurs naturally in stars. Fission is the opposite: breaking down a heavier atom into two or more lighter ones; it is an artificial process (e.g. in atomic bombs).
Although fission can be accomplished fairly easily, there are problems in harnessing this for energy, which will be required when fossil fuels run out. The challenge is to produce safe, controllable nuclear energy without radioactive radiation and dispose of waste products safely.