This is potentially huge
http://www.npr.org/blogs/health/2012/08/01/157706171/lab-findings-support-provocative-theory-on-cancer-enemy-within
And what a small world this is... one of the quoted physicians has treated a family member.
It is a breakthrough, despite this kind of discovery has been reported, within the various cancer types, for years now.
The use of the term "stem" is a little weird, and it has nothing to deal with the stem cells, embryonic or somatic (adult) that generate and re-generate our tissues.
A cancer stem cell is, by definition, is a cancerous cell that has the unique property, in an experimental setting, to transfer the tumor from the host mouse to a recipient. Differently from the normal stem cells, whose the product of differentiation into a normal tissue, the cancer stem cells "differentiate" (it's a kind of inappropriate, but it cannot be defined better) into highly proliferating tumor cells.
All in all, the cancer stem cells are the cells that are responsible for relapses after an apparently successful chemotherapy. Their rate of proliferation is very very low, so they're not sensitive to the anti-neoplastic agents, which, usually, target proliferating cells.
Accordingly, the study and profiling of cancer stem cells may provide targets for new drugs to be included in a chemotherapeutic regimen for a better tumor load clearance.