What are Stem cells ?

Stem cells in humans are primal undifferentiated cells that retain the ability to produce an identical copy of themselves when they divide (clone) and differentiate into other cell types. http://en.wikipedia.org/wiki/Stem_cell

This should explain all you need to know.

http://stemcells.nih.gov/info/basics/

Stem cells in humans are primal undifferentiated cells that retain the ability to produce an identical copy of themselves when they divide (clone) and differentiate into other cell types. In higher animals this function is the defining property of the deleted cells. Stem cells have the ability to act as a repair system for the body, because they can divide and differentiate, replenishing other cells as long as the host organism is alive.

Medical researchers believe stem cell research has the potential to change the face of human disease by being used to repair specific tissues or to grow organs. Yet there is general agreement that, "significant technical hurdles remain that will only be overcome through years of intensive research."[1]. Current evidence indicates that some stem cells are involved in assisting cancer's proliferation, or worse yet, some stem cells act as cancer stem cells (CSC).

The study of stem cells is attributed as beginning in the 1960s after research by Canadian scientists Ernest A. McCulloch and James E. Till.

Stem cells are cells that have the ability to grow into any function, for example, an eyeball or a thumb bone -- they are mostly easily obtained from fetuses, which is why studies about stem cells are controversial.