Stem cells are the developmentally early forms of flexible cell types which can give rise to multiple cell types. Embryonic and adult stem cells are the two types of stem cells occurring in the mammalian system, which are responsible for giving rise to myriads of cell types during the development. Accordingly, harnessing the potential of stem cells is considered to have great significance for therapeutic purpose. Also, stem cells have certain features like unlimited proliferation potential, which they share closely, with the rogue cells of any mammalian system, which are cancer cells. In addition, there are stem cells, which give rise to cancer and are called cancer stem cells. In this review, we have addressed the developmental aspects of stem cells, cancer cells and cancer stem cells, their respective biological functions in maintaining an intricate balance to decide their respective contributions to the usefulness or harmfulness in the biological system.