A high school reader asked, What's the difference between stem cells and regular cells?
In really simple terms, I would say the answer is that regular cells already look like what they are. If you take a skin cell or a muscle cell or a heart cell and put it under a microscope, it will look like any other skin cell or muscle cell or heart cell. If you look at a stem cell, it will look different--it has the potential to be many kinds of cells.
Cells contain DNA, which makes up our genes. We have millions of genes which are not expressed--in other words, they are inactive. For example, I have a gene for blue eyes that I got from my mom, but it is not expressed; I have green eyes. However, my son has blue eyes--I passed that gene on to him. (He also got a blue-eye gene form his green-eyed dad, whose mother also has blue eyes, otherwise it wouldn't have been expressed since it's a recessive and needs two.) We have lots and lots of other genes which are not expressed.
A cell that has differentiated only expresses the genes for the kind of cell it is. When it divides (reproduces), it makes an exact copy of itself.
A stem cell has the capacity to express many more genes. An embryonic stem cell can theoretically express anything. When a stem cell divides, it creates a copy of itself and it also creates another cell which has the potential to be many different things. Proteins and enzymes and other chemicals in the environment (the body) tell it what to become.
Bottom line--if you put a skin cell in a culture and give it lots of different things that make it grow, it won't ever become a muscle cell. If you put an embryonic stem cell in a culture and give it things, you can make it either a skin cell or a muscle cell. If you put an umbilical cord or other adult stem cell in a culture and give it lots of things, you can make it become some things but not all--for example, a blood stem cell can't become cartilage although it can become a variety of types of blood cells.
On a theoretical level, since cells contain so much unexpressed, inactive DNA I suppose you could turn any cell into any other type of cell. The technology is nowhere close, though.