Trans fats are primarily created through hydrogenation, a process that turns liquid oils into solids like hard margarine and shortening. Some foods with trans fats include vegetable shortening, some margarines, crackers, cookies, and many packaged snack foods. Some animal-based foods have small amounts of naturally occurring trans fats.
Trans fats do not spoil as quickly as liquid fats, which makes them better in foods that are packaged for a long time.
Trans fat can increase cholesterol levels the same way as saturated fat. The best way to check for trans fat in a food is to look at the list of ingredients. Food made with partially hydrogenated or hydrogenated oil will have trans fat.