The human body is composed of billions of cells that are continually ageing, dying and being replaced. Cell death, replacement, growth and development are normally tightly controlled. If this control breaks down, cells begin to grow and divide abnormally, clustering together to form a lump known as a tumour. Tumours can be benign (non-cancerous) or malignant (cancerous):

Benign tumours remain contained in one site and do not spread to other sites. Some benign tumours do continue to grow and may need to be removed. Others do not grow above a certain size or cause any symptoms and, thus, never need to be removed.

Malignant tumours have an unlimited potential to grow and to spread from the organ of the body where they started (e.g. breast, prostate, bowel) to other parts of the body. These malignant tumours are referred to as cancer. The spread of cancer from one part of the body to another is called metastasis.

The organ where a cancer originated is called the "primary cancer site" and this site gives cancer its name, like “liver cancer” or “colorectal cancer”.

Cancer that has spread from the primary site to another organ (e.g. the liver or lungs) is called metastatic or secondary cancer.

NB: Some cancers do not form tumours. For example, leukaemia is a cancer of the bone marrow and blood.