These cells reproduce themselves by dividing in a regular way so growth and repair of body tissues can take place.
Cancer develops when cells start to divide at the wrong time and in the wrong place, then continue to divide and invade nearby tissues and organs. It is this uncontrolled growth of cells that causes a swelling or tumour.
Tumours can be benign or malignant
A benign tumour is contained within a localised area and once treated doesn't usually cause any further problems.
A malignant tumour, however, can spread to nearby tissues and organs, travelling via the bloodstream or lymphatic system to other parts of the body where they may form new tumours.
These secondary cancers are known as metastases. It is because cancer cells move to vital organs, such as the lungs or the liver, and prevent them working normally that cancer anywhere in the body is potentially life-threatening.
There are around 200 different types of cancer, depending on the cell type involved, and they vary greatly from each other and in the types of treatment needed.
Pictured: Advanced malignant melanoma - photo US National Cancer Institute
