Please explain me what is nuclear fission and what is nuclear fusion? Please even tell about nuclear fission reaction .

Dear Student,
(a) Nuclear fission reactions are those reactions in which a bigger nucleus breaks into two or more smaller nuclei upon bombardment with high speed particles like neutron, with emission of alpha particles/beta particles/ gamma rays. These reactions too produce large amount of energy and are used in the nuclear power plants to generate electricity. They can result into a chain reaction, due to production of additional particles.

For example, Uranium disintegrates into Barium-141 and Krypton-92 upon bombardment with 1 neutron and emit 3 more neutrons.
$U^{235}+ n^1 \to B{a}^{141}+K{r}^{92}+3n^1$

(b) Nuclear Fusion reactions are those reactions in which two small nuclei fuse together to form a bigger nucleus, with evolution of large amount of energy (almost three to four times greater than nuclear fission reaction). These type of reactions occur in the Sun and are responsible for the heat energy of the Sun. These reactions require very high temperatures.
For example, two deuterium nuclei fuse to form one helium nucleus as:
$H_1^2 + H_1^2 \to H{e}_2^4 +Energy$

Regards.