There are several differences between Crohn's disease and Ulcerative Colitis.
Here are a few: Crohn's disease can affect the GI tract anywhere from the mouth to anus; Ulcerative Colitis only affects the colon. There is a term, "Crohn's colitis" that is used to describe Crohn's Disease affecting the colon.
In Crohn's disease, the entire thickness of the bowel wall is inflamed and where the disease affects the small intestine there is swelling of the tissue that connects the small intestine to the back wall of the abdomen and contains the main intestinal blood vessels and lymph glands.
In ulcerative colitis, inflammation is usually confined to the inner lining of the colon, which becomes raw and can bleed. In Crohn's disease, fistulas and abscesses can occur.
Strictures (narrowing of the bowel) can occur in both forms of IBD, but are more common in Crohn's disease, owing to the swelling occurring throughout the thickness of the wall of the bowel rather than being confined to the inner lining ]that is usual in U.C.
Hope this helps. For further reading a very accessible digest (pardon the pun), you might like Fred Saibil's, "Crohn's disease and Ulcerative Colitis" http://www.amazon.co.uk/Cr
ohns-Disease-Ulcerative-Co litis-Everything/dp/155297 7714/ref=sr_1_4?s=books&ie =UTF8&qid=1295816549&sr=1- 4