What are the two kinds of stem cells?

Stem cells come in two varieties:

1. Blastocysts (often mistakenly called embryonic, a charged word that has created a political and moral brouhaha): When a fertilized egg turns four days old, a cluster of specialized stem cells walls itself off to create an inner cell mass. These cells have the amazing ability to reproduce indefinitely, and, if they become embryos, they have the ability to mature, grow, and differentiate themselves into every tissue that forms every piece of your bodily puzzle. But they aren't embryos yet; they cannot go on unless they implant. At this stage, they are blastocysts, or pre-embryonic. These immortal blastocystic stem cells retain their natural and quite spectacular ability to differentiate into any and every organ. In other words, they have the luxury of deciding what to become when they grow up. Do they mature into heart cells, liver cells, or brain cells? Because of their plasticity, these cells have the greatest potential to cure diseases, especially those associated with aging, such as Parkinson's.
2. Progenitor cells (also called adult stem cells): Now, some of those blastocystic cells, like kids living at home when they're 30, stay right where they are and don't mature into other tissues and organs. Instead, they hang back and set up shop in the bone marrow. These adult stem cells retain the ability to grow into other kinds of cells.

Why is this so exciting from a medical and scientific perspective? If your own stem cells—the cells you currently have—can be used to regenerate new tissue to replace broken-down or diseased tissue and fix your own organs, then you have the opportunity to punch frailty right in the face.