I had to look this up myself, but I found a site that explains the difference. It says:
Irritable bowel syndrome has similar symptoms to ulcerative colitis and Crohn’s Disease. However, ulcerative colitis and Crohn’s Disease are bowel disorders with inflammation and IBS is not.
Ulcerative colitis is when the lining of the colon becomes inflamed and develops ulcers. The chronic symptoms of this disorder may include bloody diarrhea, abdominal pain, constipation, nutritional deficiencies, food intolerances,
achiness, joint pain, itchiness, flatulence and bloating.
I believe that a colonoscopy will show physical evidence of UC, but not IBS. It is typically after other digestive disorders with physical evidence have been ruled out that a diagnosis of IBS is made.
Someone with more knowledge or experience may be able to answer this better, but I hope this helps somewhat.