Fertilization is defined as the process in which there is fusion of male gamete and female gamete.
The male gamete is the sperm that is released from the testis. Sperms are released in millions of number as it has to travel outside the male body. Thus, to increase the chances of its fusion with egg, it is released in more number. It is released along with semen which is a fluid that helps in the movement of sperms.
The female gamete is the ovum that is released from the ovary. Ovum is released once in a month as it is present inside the female body. It releases from the ovary into the fallopian tube where it stays till the sperm gets fertilized, otherwise it comes out of the body during menstruation.
During fertilization, one of the sperms fuses with the ova/egg. The nuclei of the sperm and the nuclei of the egg fuse to form a single nucleus resulting in the formation of a fertilized egg called as zygote. Zygote is a single cell having two nuclei, one from each gamete.
In human beings, the fertilisation takes place inside the female body, which is known as internal fertilisation.