The union/fusion of male gamete (Sperm) and female gamete (Ovum or egg) is known as Fertilization that produces the first little most body part of a man called a Zygote.
The act of fertilization takes place in the female genital tract that is in the Ampulla (the isthmic junction of the fallopian tube [Oviduct]) of a female body.
A sperm remain alive for 72 hours after the entering into the vagina, but its capacity to fertilize the ovum lasts for 48 hours. So, the 12th to 18th days of menstruation cycle is correct for getting pregnancy for a lady, because the Ovulation (releasing of ovum or secondary oocyte from the Graafian follicle of the ovary) happens about the 14th day of the menstrual cycle from any one side of the ovary. The chance of fertilization reduces if sperms enter into the genital tract of the female before 48 hours or 12-24 hours after ovulation.
During actual sexual intercourse/coitus [vaginal, not anal] semen is accumulated on the upper part of the vagina. [Semen is the viscous milky white heterogeneous fluid mixture that contains 100 to 200 millions of sperms together with the secretion of the epididymis, seminal vesicle, prostate gland and Cowper's gland] The smart, responsive and healthy sperms in the semen then go to the cervix, then pass through the uterus and then ultimately arrive in the ampulla part of the Fallopian tube where they come to meet the ovum if present. The sperms travel through the contractions of vagina, cervix and uterus with a velocity 2 mm per minute and then reach in the ampulla. The time taken by the sperms is 45 minutes in average to go in the ampullary-isthmic junction from the cevix. When the sperms reach near to the ovum then they surround the ovum and several chain actions and reactions take place thereafter. Although so many sperms encircle the ovum but only one sperm is allowed to penetrate the ovum!!! So think about the competition among the sperms. An ovum is covered by two layers of epithelial cells; outside Corona radiate and inside Zona pellucida. These two layers are dissolved before the fertilization. The cells of Corona radiate are joined to each other by mucopolysaccharide of hyaluronic acid polymers. It can be concluded a lady sperm keeps her affair with several sperms but gives her heart to only one, so naughty romantism!!!
Now, the allowed sperm has to face a strong fight to reach entirely inside the ovum. Firstly, it secretes an enzyme Hyaluronidase to reach the Zona Pellucida. Secondly, the fittest sperm loses its acrosomal cap and then secretes lysosomal enzyme which dissolves a specific area of the zona pellucida. Then the sperm enters through this area and then passes through perivitelline space and then penetrates vitelline membrane. During such penetration only the head containing genetic elements of the fittest sperm enters into the ovum leaving the lashing tail outside the ovum. So, think Darwin's theory of the "Survival of the fittest" here also satisfied.
After accepting the most handsome sperm the female ovum secretes a membrane around it which is called fertilization membrane. This membrane prevents the further penetration of any other sperm. So, think how much clever and intelligent the human ovum is. After the entry of the sperm inside the ovum the male nucleus from the head becomes male pronucleus and the nucleus of the ovum becomes female pronucleus. The centrosome of the sperm divides to form two daughter centrosomes and the moved to the opposite poles of the ovum. The two pronuclei fused to form a single nucleus. Now, this cell with a single nucleus is called the Zygote. Zygote then undergoes further development to forms Morula, blastula, gastrula, embryo and then a complete baby.