Crohn’s disease is a disease that causes inflammation, or swelling, and irritation of any part of the digestive tract—also called the gastrointestinal (GI) tract. The part most commonly affected is the end part of the small intestine, called the ileum.

The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. The movement of muscles in the GI tract, along with the release of hormones and enzymes, allows for the digestion of food.

The GI tract

In Crohn’s disease, inflammation extends deep into the lining of the affected part of the GI tract. Swelling can cause pain and can make the intestine—also called the bowel—empty frequently, resulting in diarrhea. Chronic—or long-lasting—inflammation may produce scar tissue that builds up inside the intestine to create a stricture. A stricture is a narrowed passageway that can slow the movement of food through the intestine, causing pain or cramps.

Crohn’s disease is an inflammatory bowel disease (IBD), the general name for diseases that cause inflammation and irritation in the intestines. Crohn’s disease can be difficult to diagnose because its symptoms are similar to other intestinal disorders, such as ulcerative colitis and other IBDs, and irritable bowel syndrome. For example, ulcerative colitis and Crohn’s disease both cause abdominal pain and diarrhea.

Crohn’s disease may also be called ileitis or enteritis.

Ref: http://digestive.niddk.nih.gov/ddiseases/pubs/crohns/#what