Sometimes treatment for Crohn's disease, ulcerative colitis, and familial adenomatous polyposis involves removing all or part of the intestines. When the intestines are removed, the body needs a new way for stool to leave the body, so the surgeon creates an opening in the abdomen for stool to pass through. The surgery to create the new opening is called ostomy. The opening is called a stoma.
Different types of ostomy are performed depending on how much and what part of the intestines are removed. The surgeries are called ileostomy and colostomy. When the colon and rectum are removed, the surgeon performs an ileostomy to attach the bottom of the small intestine (ileum) to the stoma. When the rectum is removed, the surgeon performs a colostomy to attach the colon to the stoma. A temporary colostomy may be performed when part of the colon has been removed and the rest of it needs to heal.
Ileoanal reservoir surgery is an alternative to a permanent ileostomy. It is usually completed in two surgeries. In the first surgery, the colon and rectum are removed and a pouch or reservoir is constructed from the last 18 inches of the small intestine. This pouch is attached to the anus. In the second surgery, the ileostomy is closed. The muscles surrounding the anus and anal canal are left in place, so the stool in the pouch does not leak out of the anus. People who have this surgery are able to control their bowel movements.
If an ileoanal reservoir is not possible or feasible, a continent ileostomy may be an alternative to using an outside collecting bag. In continent ileostomy, an internal reservoir pouch is created from part of the small intestine. A valve is constructed and a stoma is placed through the abdominal wall. A tube is inserted through the stoma and valve to drain the pouch.
source: http://digestive.niddk.nih.gov/ddiseases/pubs/ileostomy/index.htm