There are three varieties of lard that can be obtained from the carcass of a hog, and each has its own particular characteristics and best uses.
Pork fatback is the fat that comes from underneath the skin on the back and shoulders. When you refer to lard generically, you're talking about this. It's got a nice porky flavor, and it's great for making sausages -- just chop it up and throw it in the grinder.
Or you can render it and use it for cooking and baking. Pork fatback makes a superbly flaky pastry. sometmes called hard fat.
Then there's leaf lard, which is a soft fat that's harvested from around the kidneys. Because there's less of it than fatback, and because it's so soft, leaf lard is something of a delicacy. It has a very neutral flavor, and when rendered it is probably the best type of shortening for making pie crusts.
The third type of pork lard is caul fat. Caul fat is a lacy membrane of fat that surround the stomach and other digestive organs. It's sometimes called fat netting, and in France it is often referred to as crépine.
Caul has a stringy texture, which makes it difficult to render (and there's not much of it in any case). and a pronounced porky flavor. Its unique construction gives it a unique function as a wrapper for sausages and forcemeats.
The classical French tradition of charcuterie, or sausage-making, features a number of items that consist of some type of filling wrapped in caul fat.
These items are called crépinettes, and they can be shaped like patties or small cylindrical bundles.
Cooking crépinettes is done on the grill usually, and the fat will melt away, by which time the forcemeat within will be cooked and will continue to hold its shape. The fat also adds moisture and flavor to the forcemeat as it cooks.
Crépinettes can also be done in a hot skillet or in the oven.
An entire caul can be used to wrap a large roulade, or even an entire roast. When employed this way, it's essentially a form of barding.
Finding caul fat can be a challenge. Your best bet is to hit up a local butcher, especially if they do their own fabricating (a fancy word in butcherese that means "cutting up"). They might not have any on hand, but if you put in a request, they'll save it for you. Chances are the butchers themselves have been taking it home for their own personal use. That, by the way, is exactly the sort of butcher you want to find.