Irritable bowel syndrome is a kind of catch-all term, IMHO, when there's a digestive issue and the doctors really don't know what's going on. The normal movement of the intestines doesn't work as it should, but there is no visible cause, no inflammation. You can read more about
Inflammatory bowel disease is much more serious. It's thought to be an immune disorder where the body attacks itself, or some unknown invader, causing inflammation, ulceration and bleeding. There are two diseases: Crohn's disease which can strike anywhere in the gastrointestinal tract and ulcerative colitis, which affects only the inner lining of the colon.http://www.webmd.com/ibd-crohns-disease/default.htm
As Tab has indicated, some doctors believe it is possible to have both conditions simultaneously. My personal opinion is it's more of a continuum, rather than separate conditions. If your doc has seen ulceration during your colonoscopy, it sounds like you do have inflammatory bowel disease, although it could either by UC or Crohn's colitis.