Fiber is also a chain of sugars, however, the enzymes in our digestive tract aren't able to break these down. Their role is mechanical--they help the digestive process move forward. If the fiber comes in a food form, it is called dietary fiber. If the fiber has been taken from other sources and added to food, it is called functional fiber.
Soluble fiber is found in plant matter. The word "soluble" means it can be dissolved in water. Once this fiber is introduced to any liquid, though, it turns kind of jello-like. The bacteria in the inestines are capable of digesting this matter, although a by-product of this process causes gas to be released, which then builds up and is released in ways that we say, "excuse me" afterwards. Some of this fiber can be absorbed as nutrition, but most of it serves to "bulk up" the waste matter, which can help move it through.
Insoluble fiber, therefore, are the fibers that don't break down in liquid. They add to the waste bulk as well. A popular source of this sort of fiber is the outer part of a kernel of grain, called the bran. There are also veggies and fruits containing insoluble fiber.
There are a few substances which are fiber at a chemical level, but aren't classified as carbohydrates.
This is a good place to insert that the inside of the intestines are dark, humid, and warm places. Both good and bad bacteria love this sort of environment. The intestines are working to absorb nutrients and water in the liquid matter that is digesting. However, keeping the matter moving is important. This is called "waste" for a reason, and the intestines aren't meant to be dumpsters that store everything that has no useful purpose left. This is why fiber is great--it allows the transit time of the digesting matter be short.