Cancer stem cell research has drawn a lot of attention recently. The importance of cancer stem cell research lies in the possibility of providing new approaches for improved understanding of cancer biology and cancer treatment. However, cancer stem cell research is still at its infancy. In this review, we will discuss the concept of cancer stem cells and the commonly used approaches for isolating or enriching cancer stem cells. We then review the common features of cancer stem cells including resistance to cancer drugs and stresses such as hypoxia and radiation. We also summarize signaling pathways which might be preferentially important for cancer stem cells. Finally, we will propose a new Yin-Yang model of cancer stem cells and discuss the possibility of developing new drugs that target cancer stem cells for improved treatment of cancer.