According to the Wikipedia article on shortening, it seems as though margarine is considered shortening.
Has anyone substituted margarine for shortening in any recipes, and can you report your results?
Margarine is essentially 80% hydrogenated vegetable oil, the rest being mostly water and a touch of coloring and flavoring.
Shortening is essentially 100% hydrogenated vegetable oil.
That means that to truly substitute shortening versus margarine, you would need 5 units of margarine, versus 4 units of shortening plus one unit of water.
However, in many, many applications substituting 1:1 is well within the tolerance of recipes. This applies to almost all savory cooking, and most baking including breads, quick breads, cookies, and cakes.