What is Fission and Fusion?
Fission: The breaking down of a Nucleus (not atom) into minute nuclei. It is usually induced by a neutron. For instance, a Helium nucleus (known as alpha particle) is separated into two 4He(+2) --> 2H(+1) + 2H(+1).
A lot of enery is released in the process.
Fusion: This occurs when two nuclei combines to form a larger nuclei. Large amount of enery is required to start this. Because its not simple to bring two positively charged nuclei closer.