An *undifferentiated* stem cell can assume the role of any cell in the body. A differentiated cell is one that has a specific role (i.e. stomach, brain, etc.). Differentiation is when a stem cell changes into a specific 'differentiated' cell. In the normal situation, the stem cells differentiate *properly* and become normal cells of the tissue/organ in question. In the cancer, the stem cell failed to differentiate properly and has become something in between a stem cell and a normal cell, it is both and neither. It can move through the body freely, and plant anywhere like a stem cell. But it has assumed a partial role as a cell of the affected organ.
If you have read Kelley's work on trophoblasts, you see that a trophoblast is a stem cell, but in Kelley's time, we did not fully understand the mechanics.