A colostomy is a surgical procedure that involves connecting a part of the colon onto the anterior abdominal wall, leaving the patient with an opening on the abdomen called a stoma. In a colostomy, the stoma is formed from the end of the large intestine, which is drawn out through the incision and sutured to the skin. After a colostomy, feces leave the patient's body through the abdomen. A colostomy may be permanent or temporary, depending on the reasons for its use.

There are many reasons for this procedure. Some common reasons are:

A section of the colon has been removed, e.g. due to colon cancer requiring a total mesorectal excision, diverticulitis, injury, etc, so that it is no longer possible for faeces to pass out via the anus.

A portion of the colon (or ileum) has been operated upon and needs to be 'rested' until it is healed. In this case, the colostomy is often temporary and is usually reversed at a later date, leaving the patient with a small scar in place of the stoma. Children undergoing surgery for extensive pelvic tumors commonly are given a colostomy in preparation for surgery to remove the tumor, followed by reversal of the colostomy.

From http://en.wikipedia.org/wiki/Colostomy