Natural casings can be pork or lamb. There are also collagen, cellulose (fibrous), and plastic casings. The type of casing used depends on the type of product being made and the size of the stuffed product. I use collagen casings for bratwurst and breakfast sausages that are about an inch in diameter. Natural casings are thinner and break easier, so they are used for smaller sausages. Plastic casings are used for large diameter sausages like balogna. Natural and collagen casings are edible, but cellulose and plastic casings are not, but the latter are strong enough to support large sausages. Sometimes bladders and stomachs are used for specialty meats - you probably wouldn't see them in your grocery store.