Margarine and shortening are liquid vegetable oils made creamy when manufacturers convert some of the unsaturated fats into saturated ones.
Hydrogenation hardens a liquid vegetable oil so it looks, feels, and acts like butter.
Consequently, the processed oil doesn't ooze out of cookies or corn chips, spreads on an English muffin, and is the current fat of choice for frying everything from chicken to french fries at most fast-food restaurants. But hydrogenation does much more than just add a few hydrogens here and there. These unsaturated trans fatty acids are the problem. Any or all of these changes increase heart-disease risk. The potential health risks of TFAs wouldn't be an issue except that we're eating a lot more of them than we think.