Cancer is becoming the leading cause of death all around the world. To develop better therapeutic options against cancer, we need a thorough understanding of tumor development and dissemination. As our knowledge increases, it becomes apparent that cancer is a very complex disease and this complexity is partially due to the great level of heterogeneity even within the same tumor mass. Therefore, there is a pressing need to decipher complex regulations and interactions of the tumor cells that lead to different hierarchies. Concepts of tumor-initiating and self-renewing stem cells have long been proposed to explain the emergence of a vast number of progenies within monoclonal neoplastic growth. It is now known that cancer stem cells which are found in many cancers have many roles in tumor development and dissemination. Many fascinating properties of cancer stem cells draw further attention to clarify their involvement in tumor cell plasticity, epithelial to mesenchymal transition, chemotherapy resistance and to develop therapeutic strategies for their targeting. Here we summarized recent efforts to illustrate the progress in our understanding of the biology of cancer stem cells.