Zygote is the name given to the ball of cells that will eventually become an embryo, and ultimately a foetus. It is formed shortly after the fertilization of the woman's ovum (or egg).
From the many millions of sperm that make their way to the woman's ovum, it takes just one to fertilize the egg. As soon as the sperm penetrates the ovum, the tail of the sperm separates from the head, a cell begins, and the zygote is formed.
Once created, the zygote then divides into two cells, then four, eight, sixteen and so on. As the zygote continues its journey along the fallopian tube, it continues to divide and multiply cells, although the zygote does not actually increase in size.
After around four days, the zygote becomes known as a morula, and is basically a solid cluster of cells. A week after fertilization, this dense ball of cells reaches the uterus.
Now known as a blastocyst, this cluster of cells starts to attach to the uterine wall, and embed deep into the lining of the womb. This process is known as implantation.
Once implanted, the blastocyst will begin to develop an outer layer, which will become the placenta; and an inner layer, which will become the embryo.