The appendix is a small dead end pouch, like a little tube, that comes off the caecum. The caecum is the first part of the large intestine (large bowel) just before the colon. The small intestine digests and absorbs food. The parts of the food that are not digested begin to be formed into faeces (motions) in the caecum.
The appendix is normally about 5-10 cm long and quite thin. The appendix appears to have no function. The reason it is there is a bit of a mystery.
Appendicitis means inflammation of the appendix. The inflamed appendix becomes infected with bacteria (germs) from the intestine. The inflamed appendix gradually swells and fills with pus. Eventually, if not treated, the swollen appendix might perforate (burst). This is very serious, as the contents of the intestine then spill into the abdominal cavity. This can cause a serious infection of the membrane that lines the abdomen (peritonitis), or an abscess in the abdomen. So, if appendicitis is suspected, early treatment is best before it bursts.