Stem cells aren't differentiated cells. A fetus starts as a bunch of cells that are destined to become cells of the body. Some cells will eventually be led to express the proteins to become skill. Others will become muscle, other bones, etc, etc. Thus they will be differentiated into skin, muscle, bone, etc. However, there are many levels of differentiation. Fetal stem cells for example can become programmed to become anything, whereas adult stem cells may be partially differentiated so that they may become a set of cells. For example, connective tissue stem cells can be told by the body to become skin, fat, tendon, etc.