A hernia is a protrusion of a viscus (an organ) beyond its normal limits. It can be thought of as a hole in the muscles of the groin through which the abdominal contents can protrude. This is usually fatty tissue (the omentum) but it can be bowel and rarely even the bladder can be found in a hernia.
The protrusion produces a swelling. If the swelling clears on lying down, it is known as a reducible hernia, or if it fails to clear it is known as an irreducible hernia.
Hernias can either be painless, cause slight discomfort or be painful. They tend to get larger with time and can make it difficult to undertake normal activities.