A stem cell
is a non-specialized, generic cell that can make exact copies of itself indefinitely and can differentiate and produce specialized cells for the various tissues of the body. There are two types of stem cells: adult and embyonic. Embryonic stem cells can be obtained from either aborted fetuses or fertilized eggs that are left over from in vitro fertilization. Adult stem cells are not quite as versatile for research purposes because they are already specific to certain cell types.