A stem cell is essentially the building block of the human body. The stem cells inside an embryo will eventually give rise to every cell, organ and tissue in the fetus's body. Unlike a regular cell, which can only replicate to create more of its own kind of cell, a stem cell is pluripotent. When it divides, it can make any one of the 220 different cells in the human body. Stem cells also have the capability to self-renew -- they can reproduce themselves many times over.

There are two types of stem cells: embryonic stem cells and adult stem cells. Embryonic stem cells come from an embryo -- the mass of cells in the earliest stage of human development that, if implanted in a woman's womb, will eventually grow into a fetus. When the embryo is between three and five days old, it contains stem cells, which are busily working to create the various organs and tissues that will make up the fetus.

Adults also have stem cells in the heart, brain, bone marrow, lungs and other organs. They are our built-in repair kits, regenerating cells damaged by disease, injury and everyday wear and tear. Adult stem cells were once believed to be more limited than stem cells, only giving rise to the same type of tissue from which they originated.