Nuclear fission

Here fission means"to split up "and to" break down into two or more parts. So we can define nuclear fission as :- when a low energy slow moving neutron is bombarded to heavy nucleus of radioactive atoms which is highly unstable then its nucleus split up into two smaller nuclei with a tremendous amount of energy. ( some masses which lose during this reaction then it change into energy.)

Nuclear fusion

Here the word fusion means to join or to combine. This is a process in which two lighter element nuclei like Hydrogen combine to form a heavy nucleus like that of helium is called nuclear fusion. for the fusion process a lot of initial heat energy and high pressure are required to force the lighter nuclei to fuse together to form a bigger nucleus during this process a lot of masses losses and appeared as a tremendous amount of energy. The sun which give us heat and light derives its energy from the fusion of hydrogen nuclei into Helium nuclei with going on inside it all the time.

Hope it help u

Nuclear fusion --- It is a nuclear process, where energy is produced by smashing together light atoms.
Nuclear fission --- In nuclear fission the nucleus of an atom breaks up into two lighter nuclei.