Benign tumor cells have in common with cancer cells that they divide too much. The cells in a benign tumor at least respect boundaries, though, so they are nowhere near as dangerous as cancer cells. For instance, cells dividing to form a tumor on the surface of skin would not cross the basement membrane if they were benign but would cross the basement membrane if they were malignant. A single-celled organism cannot have cancer. For example, an ameba would not get cancer.
Cancer is a disease of tissue, of cells interacting with other cells in an abnormal way.
In other words, cancer is a disease of cell sociology.