A cancer is an uncontrolled proliferation of cells. Cancers are clones . No matter how many trillions of cells are present in the cancer, they are all descended from a single ancestral cell.
Cancers begin as a primary tumor .
Metastasis is what usually kills the patient. Cancer cells are usually less differentiated than the cells of the tissue where they arose. Many people feel that this reflects a process of dedifferentiation , but I doubt it. See Cancer Cells in Culture for other properties of cancer cells. Their mutated or over-expressed products stimulate mitosis even though normal growth signals are absent.