IBD actually causes functional damage to the bowel wall IBS doesn't. Actual symptoms can be similar eg diarrhea / constipation etc but IBD is more serious due to the organic damage to the intestines. IBD causes inflammation, ulcers, thickening of the tissue which can cause strictures, fistulas etc IBS won't physically change the tissue. Thats a very simplistic overview and I'm sure someone will describe it better!!