Slo said:
I need charts and graphs!
Ehhh, I got nothing here. But in layman's terms (which is as far as I could go) there are different types of stem cells which are good for various purposes by the body; able to become different sorts of other cells. The controversial embryonic ones, though, are like the purest and most flexible types, able to be coaxed to do anything you'd want a stem cell to do.
Here's a horrible analogy, since I love them. We might think of an embryonic stem cell as being the letter A, and able to be turned into any A word from Abracadabra to Alakazam. If you can get a stem cell that's already to the point of "Ac" that's not as useful, though you can still find things to do with it if you need Action, Accurate, or Acme. If you need Advil, though, you're screwed.
I've heard that there's some reason to believe some of the less controversial ones could be made to revert or be made more flexible than we know how to now, but for now they're not as useful.