I had never heard of soluble fiber before until someone mentioned they heard soluble fiber helped decrease cholesterol levels. Found mostly in oat bran, melons, and dried fruits. Soluble fibre helps reduce cholesterol levels by eliminating bile. Cholesterol is used to make bile and upon digestion of food bile is released. Bile binds to soluble fibre, but since the fibre can't be broken down, it and the bound bile are excreted in the feces.
Insoluble fibre is the 'roughage' that helps keep things moving.
Fiber Cellulose or fiber, the structural part of plants, is not properly a nutrient, since it is neither digested nor absorbed. It is now thought that the fiber found in whole-grain cereals, including the hulls called bran, is of greater importance to the intestine than that in fruits and vegetables. In addition to providing bulk, cereal fiber has the capacity to absorb water, keeping the stool moist and soft as well as bulky. Unfortunately, fiber is one of the first parts of a whole food to be removed in the process of refining. In the United States, flour has been roller-milled since about 1880, and most of its bran is removed in the process.