Different mechanisms contribute to intratumor heterogeneity, including genetic mutations, the microenvironment, and the existence of subpopulations of cancer cells with increased renewal capacity and the ability to recapitulate the heterogeneity found in primary tumors, which are referred to as cancer stem cells (CSCs). In this review, we discuss how the concept of CSCs has been defined, what assays are currently used to define the functional properties of CSCs, what intrinsic and extrinsic mechanisms regulate CSC functions, how plastic CSCs are, and the importance of epithelial-to-mesenchymal transition in conferring CSC properties. Finally, we discuss the mechanisms by which CSCs may resist medical therapy and contribute to tumor relapse.