Meat Basics 101 states that sausage casings can be made using animal intestines, processed collagen or non-edible casings intended to be removed before eating. Loose sausage, such as is used for sausage patties, requires no casing at all.
According to Meat Basics 101, natural casings are made from thoroughly cleaned and rinsed animal intestines, primarily from lamb, pig or beef. They are completely edible and are meant to be consumed along with the sausage.
Different animal intestines, as well as different intestinal segments, are often specified for use in particular sausage varieties. Lamb casings, which are delicate, are used for small fresh sausages as they cannot withstand the rigors of smoking. Hog casings are used for many varieties of sausage, including chorizo, kielbasa or bratwurst. Beef casings are used for the largest sausages, such as bologna or cotto salami. Collagen casings are made from beef skin. The hair is removed, the skin is cooked down and emulsified, and the resulting product is rolled into thin sheets and dried. It is completely edible and is meant to be eaten with the sausage. The size is customizable, so it can be used for any sausage preparation. Non-edible casings, like paper, are meant to hold the sausage while curing but are removed before consumption.