The following is an extract from the leaflets I was given before I had my permanent ileostomy:
An ileostomy is an opening (or stoma) of the small bowel onto the abdomen so that the motions then empty into an attached bag. It is performed when all the large bowel has to be removed. This is usually necessary when an inflammatory disease (such as
ulcerative colitis or
Crohn's disease) seriously damages the large bowel. It is occasionally performed when there is more than one cancerous growth in the large bowel. When it is performed for inflammatory bowel disease, ileostomy may be permanent. On occasion, it may be used in the emergency situation when most of the large bowel is removed but enough is left to allow reconnection of the intestine at a later date.
A colostomy is an opening (or stoma) of the colon or large bowel onto the abdominal wall so that the motions then empty into an attached bag. A colostomy may be permanent or temporary. It is permanent in certain situations with cancerous growths of the rectum. This is because removal of the cancer may mean that there is insufficient rectum (or back passage) left to be joined to the bowel. Temporary colostomies are used as a safety valve to divert the motions away from a newly joined part of the intestine. This gives the new join time to heal. It may also be used in the emergency situation where there may be a blockage of the bowel.
So what that means (to me anyway) is that the output from an ileostomy can be quite loose at times although there are foods which thicken it up (but we always have to wear an open-ended bag - with a closure of course!) whereas colostomy output is more like a regular poop (because you still have your large bowel)!