The difference between "totipotent" and "pluripotent." A totipotent cell can become any kind of cell. A pluripotent cell is limited to becoming a certain range of cells. Even a totipotent cell is not necessarily identical to a one-celled embryo - it will not put into play the self-directions to develop into a complex organism - unless it is manipulated to so. That's why, in order to clone (so far) you have to take a cell nucleus and insert it into an denucleated egg, and then stimulate the egg.
Don't bet that pluripotent stem cells won't soon be converted into totipotent ones. Researchers have already figured out how to grow eggs from male embryonic stem cells. And how to fertilize an egg using half the genetic material from any cell in the body. And one of the proposed methods for using adult stem cells to create new organs or specialized cells to treat medical conditions, involves putting the genetic material from an adult stem cell into an egg (either natural, or artificially produced) which has been stripped of its original genetic material, and then coaxing the egg to start developing as an embryo. When this technique is refined to the point where it can produce any needed organ or cell type, you can be sure it will also be at the point where it can produce a baby who is a clone of the adult whose stem cell was used. At which point stopping the embryo from developing into a baby, by steering it to grow only into the desired organ or cell type, is no different from stopping any other embryo from developing into ababy, in order to use it for some other purpose. We'd all better get used to it, because this technology will arrive in our lifetimes.