Cells are the smallest living units of living system. Stem cells are undifferentiated biological cells that can differentiate into specialized cells and can divide to produce more stem cells. Embryonic stem (ES) cells are the inner cell mass of a blastocyst, an early-stage embryo. (Thomson et al., 1998). ES cells are pluripotent and give rise during development to all derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm. Adult stem cells, also called somatic stem cells, they are stem cells which maintain and repair the tissue in which they are found (Behrens et al., 2014). Pluripotent adult stem cells are rare and generally small in number, but they can be found in umbilical cord blood and other tissues. Embryonic stem cells can be grown relatively easily in culture. Adult stem cells are rare in mature tissues. Cell potency is a cell's ability to differentiate into other cell types. Stem cells resembling totipotent blastomeres from 2-cell stage embryos can arise spontaneously in the embryonic stem cell cultures. (Macfarlan et al., 2012). New research related to multipotent cells suggests that multipotent cells may be capable of conversion into unrelated cell types. In one case, fibroblasts were converted into functional neurons. Stem-cell therapy is the use of stem cells to treat or prevent a disease or condition.