The wall of the intestine has three layers. The middle layer is the only one with much strength. Where there is a defect in the middle layer, the small amount of pressure of the intestinal contents can cause an outpouching of the wall, a diverticulum. There may be a number of them, with no symptoms. That's diverticulosis. But indigestible fiber, for instance, may lodge in a diverticulum, and cause inflammation, sometimes very troublesome. That's diverticulitis.