In order for cells to become whole organisms, they must divide and differentiate. Cells divide all the time. That means that just one cell, a fertilized egg, is able to become the trillions of cells that make up your body, just by dividing. Those trillions of cells are not all the same though.
Just a little while after you started out as a fertilized egg, your cells started performing specific tasks, even started to look different because of that. Differentiation means that one cell performs a different function than another cell, depending on where it is in your body.
Not sure if this will help.