It is either soluble or insoluble and it is either crude or dietary. Let's look at the differences between soluble and insoluble fiber first. While it isn't generally effective in the treatment of intestinal problems, it does appear to lower blood cholesterol levels and retard the entry of glucose into the bloodstream. It absorbs its weight in water many times over, swelling in the intestines. It occurs on the outside of many seeds, fruits, legumes and other foods.
Wheat bran is an example of insoluble fiber.
It is generally the chewiest part of the food in its natural form. With wheat products it's absence is the reason that white bread is generally softer than whole wheat or whole grain breads. In western cultures we often remove the natural fiber from our foods by peeling, milling and other processes which occur in commercial food processing. Insoluble fiber is key in the treatment of constipation and other intestinal disorders. It has also been linked to the prevention of colon cancer.