Stem cells are those cells which can regenerate themselves through fission.
Some stem cells can also differentiate into different number of other cells.
The embryo in the uterus of mother can be called a multicellular structure in which there are many stem cells.
These stem cells regenerate, and then differentiate to form brain, liver, lungs, and all the other organs. Thus, some group of stem cells differentiates to perform a specific function, while the same happens for another group to perform another function.