Fertilization means the fusion of male gamets and female gametes.It is a complex process and it has dual function. After fertilization zygote is formed which is the primary cell of animal and it contains diploid number of chromosomes but each gamet carries haploid number of chromosomes.
Fertilization always occurs in aquatic medium(water or body fluid).If it occurs outsides the bodies of male and female parents it is called external fertilization and if it occurs inside the body of female's body it is called internal fertilization.