An overgrowth of the inner skin lining of the bowel or anus.  Polyps can be tiny or very large.  They can also be benign or malignant.  Sometimes polyps run in families but we think that most are caused by environmental damage causing different genetic mutations in the lining of the bowel as people age.  Most polyps give no symptoms at all but sometimes they can cause anaemia, bleeding or interruption to the normal flow of faeces.