What is an omentum? How is it related to a "beer belly"?

They are very different. The omentum is part of the lining on the inside of the abdominal cavity that attaches the stomach to various other abdominal organs. A "beer belly" is very different as it is not due to a process within the abdominal cavity. A "beer belly" is basically increasing abdominal size or girth due to deposits of fat beneath the skin in large amounts, often accompanied by weakening of the abdominal muscles due to insufficient exercise. So while the omentum is a normal part of anatomy, a beer belly is increasing size of the abdomen usually due to increasing calorie intake and lack of enough exercise.