a stem cell is a cell whose function has not been decided. stem cells have the potential to become any type of cell that exists, at least in humans. every person has stem cells, but the older you are, the harder it is to find these cells, because less of them exist. the most easy place to get these cells from is the BLASTOCYST form of fetal development; herein lies the bige controversy of embryonic stem cell research
an example is someone who has diabetes. this disease causes cell in the pancreas to stop functioning and eventually die. stem cells have the potential to repair the problem before it even happens.