Cellulose is a polysaccharide (complex carbohydrate) and is the most common organic compound on Earth. About 33% of all plant matter is cellulose.
Cellulose is what provides structure, rigidity and strength to the cell walls of nearly all plants. Humans are unable to digest cellulose, it is this undigestible cellulose that provides fiber necessary for a healthy intestinal tract and contributes mightily to the joy of the "perfect poo."
Some animals, particularly ruminants, (animals that chew their cud) can digest cellulose with the help of microorganisms that live in their guts. Humans can't digest it as well, which is why it is referred to as "dietary fiber" or "roughage." Vegetables and legumes contain cellulose and are therefore excellent sources of fiber, these can be consumed as much as desired and are considered very Good Decisions.
Dietary fiber increases your feelings of fullness and satiety, which means your hunger level decreases. This is a huge boon for weight loss as it reduces food intake at meals. In addition, because the food is more gradually absorbed it slows the entrance of glucose into the blood stream, thereby preventing large blood glucose and insulin spikes. A diet high in fiber also contributes to a healthy digestive tract, which increases the elimination of unwanted substances from the body.
Celluloses can also be used instead of refined products to thicken a sauce or add depth and texture to a dish. Pureeing a peach for instance then adding it to your pie instead of sugar or flour will not only improve the health of the pie but make it "stand up" when sliced.