Normally the rate of cell division in the body is carefully controlled, so cells divide only when new cells are needed. Cancer is the result of changes in cells that lead to uncontrolled cell division. A cell with these changes keeps dividing by mitosis when it should not.
The cancer cells divide again and again and form a lump of cells called a tumour. Some tumours stay in one place. Others can spread to different parts of the body and form new tumours there.
The changes happen in the DNA of a cell. They can be caused by things such as ultraviolet radiation, tobacco smoke and some chemicals, although sometimes they happen by chance.