what is nuclear fusion?

In nuclear physics, nuclear fusion is a reaction in which two or more atomic nuclei come close enough to form one or more different atomic nuclei and sub-atomic particles. The difference between the mass of the reactants and products is manifested as a large amounts of energy. 
       The release of energy in the fusion of lighter elements is due to interplay of two opposing forces : the nuclear force and coulomb force.
       Naturally occurring - our Sun.
       Man made - nuclear fusion reactor e.g., wendelstein 7-x.