Shortening is a fat used in cooking. Modern shortening is a vegetable-based product that is just barely solid at room temperature. Shortening has a higher smoke point than butter and margarine (it is less flammable), leading to its use in deep-fat frying and as a pan coating to prevent baked goods from sticking. It is also sometimes used as an ingredient, when a recipe needs a flavorless fat thicker than oil but thinner than margarine. Shortening has 100% fat content, compared to 80% for butter and margarine.
Refrigeration is a good idea, because the pastry-like foods commonly made with shortening will turn out best if everything (ingredients, air, rolling pin, work surface, bowls, etc.) is kept cold until baking.
Until recently, shortening was almost always made of partially hydrogenated vegetable oil. Partially hydrogenated oil contains trans- fats, which have been implicated in causing heart disease. Newer products avoid trans- fats by using a mixture of unhydrogenated oil and fully hydrogenated oil. All vegetable-based shortenings are cholesterol-free.
Butter or margarine, mixed with vegetable oil to thin it, can replace shortening used to prevent baked goods from sticking or used as an ingredient. One way is to use lard or bacon grease, as was tradition. Lard is a particularly good substitute, producing the very best pastry crusts while having less saturated fat than butter.