A body is made up of millions of cells. Those cells divide constantly. Because at the same time cells die, there is a kind of balance. Cancer upsets this balance. One of the cells breaks up and keeps on dividing, which means that in no time at all there are way too many cells. That bad (malignant) cell is called a cancer cell.
Under the microscope the difference between a cancer cell and a healthy cell is clear to see. Cancer cells take the place of healthy cells and have the tendency to keep on growing into the surrounding tissue and organs. They can also wander around the body and settle in other places. Something has to be done about it.