Cancer -- the disease caused by an uncontrolled division of abnormal cells in a part of the body
Leukemia -- a malignant progressive disease in which the bone marrow and other blood-forming organs produce increased numbers of immature or abnormal leukocytes. These suppress the production of normal blood cells, leading to anemia and other symptoms.
Having many different kinds it can be aggressive or passive. Sometimes all you need is monitoring while in aggressive cases chemo is needed. With advancement in technology stem cell transplants are rising in popularity due to the amazing performance.