Fiber is the broad name given to the things we eat that our bodies cannot digest. The three fibers we eat on a regular basis are:
Hemicellulose is found in the hulls of different grains like wheat. Bran is hemicellulose. Cellulose is the structural component of plants. It gives a vegetable its familiar shape. Pectin is found most often in fruits, and is soluble in water but non-digestible. Pectin is normally called "water-soluble fiber" and forms a gel. When we eat fiber, it simply passes straight through, untouched by the digestive system.
Cellulose is a complex carbohydrate. It is a chain of glucose molecules. Some animals and insects can digest cellulose. Both cows and termites have no problem with it because they have bacteria in their digestive systems secreting enzymes that break down cellulose into glucose. Human beings have neither the enzymes nor these beneficial bacteria, so cellulose is fiber for us.