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.