The google definition of Stem Cells :

‘An undifferentiated cell of a multi cellular organism which is capable of giving rise to indefinitely more cells of the same type, and from which certain other kinds of cell arise by differentiation.’

The google reason why stem cells are controversial is :

The stem cell controversy is the consideration of the ethics of research involving the development, use, and destruction of human embryos. Most commonly, this controversy focuses on embryonic stem cells. Not all stem cell research involves the human embryos.