Ulcerative colitis (Colitis ulcerosa, UC) is a form of inflammatory bowel disease (IBD) that causes inflammation and ulcers in the colon.The disease is a type of colitis, which is a group of diseases that cause inflammation of the colon, the largest section of the large intestine, either in segments or completely.he most common disease that mimics the symptoms of ulcerative colitis is Crohn's disease, as both are inflammatory bowel diseases that can affect the colon with similar symptoms. It is important to differentiate these diseases, since the course of the diseases and treatments may be different.