Irritable bowel syndrome, which is commonly abbreviated IBS, is a common disorder that affects the large intestine. According to MayoClinic.org, one in five American adults will experience this condition. IBS is a chronic condition that causes abdominal pain, severe cramping, and sudden changes in bowel movements. Unlike with inflammatory bowel diseases (including Crohn’s disease and ulcerative colitis), the actual structure of the bowel is not abnormal.