Cancer is not a single, static disease. There are hundreds of types of cancer classified according to the organ in which they were created.  This includes the type of cell that is no longer controlled by the rest of the body. It can be said that a tumor is a group of cells that divide uncontrollably.  They form a tissue different from the organ from which they originated. Tumors are not always malignant and not all cancers are found in a particular organ. There are tumors that are non-solid leukemia being one example.