A zygote is the fertilized cell that will grow into a baby animal. When a female's ovum and a male's sperm cell join, the cell that results is called the zygote. The zygote then multiplies, and grows into an embryo. So, a zygote is formed from the union of two gametes, and is the first stage in a organism's development. Zygotes are produced by fertilization between two haploid cells, the ovum and the sperm cells, which make a diploid cell. Diploid cells have copies of both parents' chromosomes and DNA.
Some animals keep the zygote in their bodies until it is a full-grown baby. The time between the forming of the zygote and the baby's birth is called pregnancy. Other animals do not keep the zygote in their bodies, but lay an egg. The zygote grows inside the egg until it is ready and a larva or baby animal hatches.