What is diverticulitis vs osis?

Bad/Not-So-Bad. Diverticulosis is the condition where little pockets form on the colon at natural weak points. This is very common in the western world, presumably secondary to our diet, and increases with age. Diverticulitis is the condition where one of these pockets ruptures, leading to the release of intestinal contents into the abdominal cavity, causing (sometimes severe) inflammation and pain.
Itis= inflammation. Diverticul-osis means there are pocket-like protrusions of the colon wall, related to a low-fiber diet. This usually causes no symptoms. Diverticul-itis means there is inflammation of at least one of these pockets, and is treated with antibiotics.