What exactly is "vegetable shortening"?
In baking, the term "shortening" alone is used to mean any fat; "vegetable shortening" is a fat made from vegetable oil to be solid at room temperature.

Most vegetable oils, such as corn oil, peanut oil, soybean oil and so on are liquid at room temperature because they are unsaturated fats:  their fatty acids do not have hydrogen bound to them.

Vegetable oil is converted into vegetable shortening by hydrogenating it, forcing hydrogen to bind on to the ends of the fatty acids.  This is done by forcing hydrogen to bubble through the oil under pressure, heat, and in the presence of a catalyst.

  

Compared to vegetable oil, vegetable shortening is solid at room temperature, white in color, and much less prone to rancidity as it is a saturated fat.  It has essentially an unlimited shelf life.



It is also very neutral in flavor, so is often used to grease pans. However, any fat will do for that purpose.

In baking, it performs very well in making North American style pie crusts, where it helps promote a flaky crust, but it has little flavor.  It also performs very well in deep frying.

As a solid fat, like butter, it can also be creamed with sugar to help leaven baked goods, although it does not give the flavor benefits of butter.

Note that margarine is essentially an emulsion of vegetable shortening (about 80%) and water (about 20%) plus colors and flavorings, meant to imitate butter.



See also:

Are there any substitutes for Shortening?