Zygote is the structural and functional unit of life. This is a single cell which continuously undergoes cell division to give rise to embryo.
Adult human body is a machine in which each part is specialised for its intended function. However, there are some organs which undergo wear and tear periodically and which is required to be repaired and replacement of lost cells. For this purpose, every organs retain a small population of cells (reserved) that do not differentiate. When needed these cells proliferate, repair, renew and regenerate the organs. And these reserved cells are known as stem cells.