The egg and sperm cells are gametes. A gamete is a special cell created for reproduction. It is known as a haploid cell, meaning it contains a single set of unpaired chromosomes instead of paired chromosomes seen in most cells. Gametes contain only half the number of chromosomes of normal cells.
If an animal's normal cells such as skin and organ cells contain 48 chromosomes, the gametes would only contain 24. Gametes are designed to pair with another cell to form the beginning of a whole organism.
Consequently, When a sperm and an egg combine, the new cell contains twice the chromosomes that each gamete had. The new cell is usually known as a zygote in animals, and that cell is the beginning of a completely new organism.