Our body is composed of many different kind of cells. Blood cells and skin cells, brain cells and liver cells, each specialized for their own particular use. You cannot take a blood cell and make it do the work for a nerve cell or vice versa.
Stem cells are however undifferentiated cells. Which means that they have not yet decided what they are going to be. So stem cells can give rise to cells like themselves or any other cells in the body. The body uses stem cells to replace the old and ageing tissues of the body.