(a) Nuclear fusion is the process where two or more nuclei collide at high speeds and combine to form a stable atomic nucleus whereas nuclear fission is the process of splitting of an unstable atomic nucleus into two lighter and stable nuclei.
(b) (i) Nuclear fission provides less energy which we are able to control and prevent it from creating too much energy at the same time. That's why nuclear fission is used for the production of electricity.
(ii) On the other hand, Nuclear fusion releases a lot of energy which we are not yet able to control. Nuclear fusion is no problem when devising destruction itself. So, it is used for making a hydrogen bomb.