Try and get through it, but if your head hurts just thinking about it, skip to the third paragraph. Trans fatty acids are isomers of the normally occurring cis configuration. Before your eyes roll back in your head, this simply means that fats are mostly long carbon chains hooked together and the hydrogen atoms are on different sides of the carbon chain in trans fatty acid but on the same side in a cis fatty acid. Both are monounsaturated fats, but trans fats have different physiologic effects.
Softer margarines have fewer trans isomers, and a few are formulated without any.
Trans fatty acids have many useful functional properties in foods. They are not there because the manufacturers don't care about our health. If you want a cracker that does not go rancid during the summer or leaves a greasy stain on the pantry shelf, you cannot make it with a highly unsaturated fat like soybean oil. You either use lard or hydrogenated vegetable oils. Soybean oil is the most commonly used oil in the world, and two-thirds of the soybean oil used in the U.S. is hydrogenated.