Cancer is a general term used to describe cells that grow and spread around the body in an uncontrolled way. Our body is made up of millions of tiny cells which replace themselves whenever necessary. For example, they replace themselves when they get worn out, old or injured.
The cells of our bodies grow and multiply in a process known as cell division. It must be extremely tightly controlled if all the cells in our body are to grow in the right place, and for all our organs and tissues to function properly. If cells divide too quickly the consequences can be disastrous.
Cancer is essentially a disease of cell division. Uncontrolled cell division can have many causes, and can happen in any type of cell in the body, but it usually results from defects or damage in one or more of the genes involved in cell division. If these genes become damaged (mutated) in some way, for example by exposure to cigarette smoke or ultraviolet radiation, the cell can start to divide uncontrollably. These defective cells can multiply to form a lump of abnormal tissue called a tumor.