All multicellular organisms have stem cells (not just humans!)
All our tissues originate from stem cells, which are the precursors to the wide variety of cells we end up with at birth.
The process by which stem cells divide to become specialised is called mitosis. Mitosis is the division of a cell into two identical daughter cells. All body cells divide by mitosis except for those which become gametes, which is just a fancy word for sperm and egg cells.
Ready for more fancy science words? The name given to the process by which cells develop more specialised functions is called differentiation. During this process, some genes are switched off, while others are expressed.