Answer:
The main difference between the two is - nuclear fusion results in fusing many smaller atoms into one, while in nuclear fission, the atom splits into two or more smaller ones.
Step-by-step explanation:
Firstly, "fusion" generally means combining many things to one, while "fission" refers to breaking down one thing into different components. Also, nuclear fission is not common in nature, while nuclear fusion does (like in stars, for example).