A stem cell is a "generic" cell that can make exact copies of itself indefinitely. In addition, a stem cell has the ability to produce specialized cells for various tissues in the body -- such as heart muscle, brain tissue, and liver tissue. Scientists are able to maintain stem cells forever, developing them into specialized cells as needed. There are two basic types:

Embryonic stem cells

- these are obtained from either aborted fetuses or fertilized eggs that are left over from in vitro fertilization (IVF). They are useful for medical and research purposes because they can produce cells for almost every tissue in the body.

Adult stem cells

- these are not as versatile for research purposes because they are specific to certain cell types, such as blood, intestines, skin, and muscle. The term "adult stem cell" may be misleading because both children and adults have them. In August, 2001, President George W. Bush approved limited federal funding for stem cell research. While stem cell research has the potential to provide major medical advances, including cures for many diseases, stem cell research is controversial.