I use natural, fibrous and collagen. You can use natural for fresh or smoked sausage, usually smaller link style sausages, breakfast links or polish depending on the size of the casing (lamb or hog) and how full you stuff them. They come packed in salt, and require a good rinse and presoaking to use them. They are a very traditional casing, but are a bit of a pain sometimes. I use fibrous for my smoked cooked salami and summer sausages. The ones I use are mahogany colored (which is for looks only), about 2 1/2 in. x 20 in. They come dry and have to be soaked in warm water for about 30 min. to make them pliable to use. I use two types of collagen, clear for fresh sausages and franks, and brown for smoked sausages, mostly snack sticks. I think the ones for smoked are a little thicker and stronger. They come in a large variety of sizes for about any kind of sausage you would want. They come kind of accordian folded up, and a hank will generally do around 10# depending on the diameter of the casing. They don't require any soaking, and in fact this can weaken them. I do spray the stuffer tube with some Pam. They seem to cost more than natural, at least what I've found, but you can really crank some sausage out into them in a hurry. Natural and collagen are edible, fibrous not. I've not used sythetic, but it is what you would find on a big red unsliced balogna.