From Scientific American:
After more than 30 years of declared war on cancer, a few important victories can be claimed, such as 85 percent survival rates for some childhood cancers whose diagnoses once represented a death sentence. In other malignancies, new drugs are able to at least hold the disease at bay, making it a condition with which a patient can live. In 2001, for example, Gleevec was approved for the treatment of chronic myelogenous leukemia (CML). The drug has been a huge clinical success, and many patients are now in remission following treatment with Gleevec. But evidence strongly suggests that these patients are not truly cured, because a reservoir of malignant cells responsible for maintaining the disease has not been eradicated.
In CML and a few other cancers it is now clear that only a tiny percentage of tumor cells have the power to produce new cancerous tissue and that targeting these specific cells for destruction may be a far more effective way to eliminate the disease. Because they are the engines driving the growth of new cancer cells and are very probably the origin of the malignancy itself, these cells are called cancer stem cells. But they are also quite literally believed to have once been normal stem cells or their -immature offspring that have undergone a malignant transformation.
More here.