Fiber is a substance found in plants that the human body cannot break down. The two types of fiber are soluble fiber and insoluble fiber. Soluble fiber can dissolve in water and is found mainly in oats, beans and fruit. Insoluble fiber cannot dissolve in water and is found in whole grain products and vegetables. In particular, soluble fiber helps to control blood sugar and cholesterol levels and insoluble fiber aids in stool formation and release. Both types of fiber are beneficial to health so it is important to eat a variety of high fiber foods.