The answer posted by Ishaat Pandit is good but not accurate cause as far as I know in CLASS 8 there are 3 types of nuclear reaction I am not trying to go explain you from my studies so I will only explain first two reactions they are nuclear fusion reaction and nuclear fusion reaction respectively
The simplest definition of nuclear fission is that we are adding two atoms (to be more accurate nuclei <plural form of nucleus>)to get a bigger atom (nucleus) and a lot of energy . This is what happens in atom bomb the energy forms we get in this reaction are mainly heat and light energy for example a blast of an atom bomb.Next comes the nuclear fission in this the opposite thing happens we breaking down a big atom(nucleus) to get two smaller atoms(nuclei) and lots of energy we see this reaction in hydrogen bomb.
Thank you.
