Both are considered to be a "chronic Inflammatory bowel disease" the difference between the two is their site of inflammation.
Chron's Disease: known to affect ANY portion of the gastrointestinal tract; from the small/large intestines, rectum, anus, stomach, esophagus, and mouth.
Ulcerative Colitis: usually only in the innermost lining of the large intestine;meaning the Colon & rectum. (hint; the word "colitis": col=colon + itis=inflmmation)