What is a stem cell?
Stem cells are cells found in most, if not all, multi-cellular organisms. These cells are characterized by the ability to self-renew through mitotic divisions and to differentiate into a diverse range of specialized cell types to form a whole organism.

Types of Stem Cells
Embryologic Stem Cell (ESC's) - These are extracted from a blastocyst (fetal tissue) and are totipotent or capable of differentiating into all parts of the human body including the placenta.  Research with these cells is illegal in the U.S. as it destroys an embryo to obtain them.  However they are fraught with difficulty as they cause tumors when injected into animals and cause rejection.
Induced Pleuripotent Stem Cells (iPSC's) - these cells are derived from adult stem cells that have been induced to go "backwards in time" and become like embryonic stem cells capable of becoming all tissues except the placenta thus being pleuripotent.  These cells are promising, but currently present problems with tumor formation and rejection by the host
Adult Stem Cells - These cells exist in all organs and tissues.  They can develop or differentiate into a specific cell type to repair and sustain tissues. (multipotent) Without these we would all die.  Example:  a cardiac stem cell with make the parts of the heart.  These are the cells that are currently being used clinically in the U.S. and are the ones being studied in most research in this country.  Cardiology is a field where a great deal of progress is being made.  In addition, artificial bladders have been constructed at Wake Forest University.
Umbilical Stem Cells (UC's or UBC's) or Amniotic Tissue Stem Cells - These cells are extracted from umbilical cord blood or from the placenta or other amniotic tissue.  They are capable of becoming any tissue or cell in the body.  They hold huge promise as they do not cause rejection or tumor formation.  They have recently become available for use in the United States. China and Panama have had great success with these cells in humans!