There are two hypotheses that explain tumor progression. The first one, the stochastic hypothesis, assumes that any cell within a tumor has the capacity to form and maintain the tumor mass. The second, the so-called hierarchical hypothesis, suggests the existence of a group of cells with a stem phenotype which, like in normal tissues, preserves tumors through a continuous production of progeny. These stem cells are in a particular niche, have a higher resistance to chemotherapy and radiotherapy, and are also capable of invading and migrating to other tissues. This review describes the cancer stem cells (CSCs), their function inside a tumor and the current knowledge about these cells.