Stem Cells

Stem Cells are undifferentiated cells that can give rise, under appropriate 
stimulation, to differentiated cells.  The extent to which they can 
differentiate and the differentiated cell types to which they give rise is 
determined, in part, from the source of the stem cells.  Stem cells can be 
obtained from very early embryonic stages, from certain fetal tissue, from 
umbilical cord blood, and from certain tissues in the adult.  Different types 
of stem cells have different potentialities.  Different research problems, as 
well as different ethical implications, are attached to each type of stem 
cell.
http - this is not a real link

National Institute of Health site on stem cells
http://stemcells.nih.gov/index.asp

Good general discussion of stem cells
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Stem_Cells.html

Very good introduction to many aspects of Stem Cells including Stem Cell Guy
http://gslc.genetics.utah.edu/units/stemcells/

Link to downloadable NIH book in PDF format.  Great and complete reference.
Stem Cells: Scientific Progress and Future Research Directions
http://stemcells.nih.gov/info/scireport/

Good site on Embryonic Stem Cells
http://www.news.wisc.edu/packages/stemcells/