Cancer stem cells (CSCs) comprise the subpopulation of tumor bulk and acquire resistant to conventional therapies and are considered as the primary tumor initiator cells. Nowadays, the tumor heterogeneity originated from CSCs, and its progenitors are accepted as a mortifying drawback in front of the cancer therapies. However, escalating knowledge gained from studies investigating the biology of CSCs will open up new frames for targeted therapies and decrease the chance of recurrence of the disease. In this review, the general understanding of CSCs and current studies were discussed briefly. Considering the latest data collected from studies of CSCs, defining the tumor heterogeneity and tumor microenvironment comprehensively will be very important to step up the cancer research.