Cancer stem cells (CSCs) are a small subset of cancer cells within a tumor that are responsible for tumorigenesis and contribute to drug resistance. The CSC displays an anchorage-independent survival, active DNA-repair capacity, and relative quiescence and is capable of self-renewing and maintaining tumor growth and heterogeneity. At the molecular level, there are several signaling pathways (e. g., Wnt/beta-catenin, Notch, and Hedgehog) to control CSC properties and alteration of these pathways has been recognized as an essential step for CSC transformation. Emerging evidence suggests that CSCs are clinically relevant. These cells are resistant to conventional chemotherapy and radiation treatment. Therefore, CSCs are thought to be the most important targets for anticancer therapy. In this review, we describe the characteristics of CSCs and how to isolate them based on some of their properties, as well as their importance in oncology.