Caul fat is a thin membrane of fat covering the intestines of a pig, cow or sheep. The most common type of caul fat used in the culinary arts is pork.
Caul fat is a translucent lace of fat, and it melts when cooked, so it provides moisture and flavor to the final product. In classical garde manger, caul fat is used to wrap forcemeats and as a natural casing for sausages.
Caul fat is sometimes used to wrap roasts, so it is essentially a form of barding.
Sausage patties wrapped in caul fat are known as crépinettes.
Also see: What is Charcuterie?