Stem cells are the foundation for every organ and tissue in your body. There are many different types of stem cells that come from different places in the body or are formed at different times in our lives. These include embryonic stem cells that exist only at the earliest stages of development and various types of tissue-specific (or adult) stem cells that appear during fetal development and remain in our bodies throughout life.
All stem cells can self-renew (make copies of themselves) and differentiate (develop into more specialized cells). Beyond these two critical abilities, though, stem cells vary widely in what they can and cannot do and in the circumstances under which they can and cannot do certain things. This is one of the reasons researchers use all types of stem cells in their investigations.