Sausage Casings can be synthetic, as in man-made, or natural, natural being the cleaned intestines of cows, pigs, lamb or sheep.
Natural casings are sold by the hank. A hank is 100 yards (91 metres) of intestine, handling anywhere from 100 to 125 pounds (45 to 55 kg) of sausage meat. Hank is not a legally defined term, however, so suppliers may vary how much they actually include in a hank. When purchasing, check to see what quantity of sausage meat they say their hank will handle. You can get smaller natural casings in smaller quantities than a hank for home use.
Natural ones come vacuum packed in brine. You soak them first, draining, resoaking, then flush with water, to get rid of excess salt from the brine. They are edible.
Synthetic ones can be edible or non-edible, and come in a variety of standard widths and lengths.
Synthetic fibrous ones are non-edible, and must be peeled off when eating the sausage. They must be soaked before using, and aren’t good for sausages that will be smoked.
Most synthetic ones are collagen, made from the inside of animal skins that is scraped off, dried, ground, dissolved, then formed into casings. These are edible.