Ulcerative colitis affects only the colon. It usually involves the rectum and can affect areas up into the colon. If it affects the whole colon, it is called pancolitis.
Crohn's disease can involve any part of the gastrointestinal tract, from the mouth to the anus.
Ulcerative colitis involves only the lining of the bowel (not the entire thickness of the bowel wall).
Crohn's disease affects the entire thickness of the intestinal wall.
Patients with ulcerative colitis and those with Crohn's disease that involves the colon have a greater risk of getting colon cancer, and this risk increases over time.