Hydrogenated oils. Meats have some transfats, but most are manmade. Transfats are made by hydrogenating oils, which turns them into solids (shortening). Transfats are used in processed foods b/c they last longer on shelf that way -- cookies/pie crusts/chips/popcorn/donuts/creamer/margarine. They raise LDL (bad) and lower HDL (good) cholesterol, which leads to heart disease. Saturated fats are just as bad!
Blocks up arteries. Trans fat is what one gets by adding hydrogen to vegetable oil. The trans fat is more solid, less "oily", and makes junk food more stable for a longer time. Trans fat is bad because it raises a person's bad cholesterol, and lowers his good cholesterol, so it blocks up arteries worse than saturated fats do. Food labels that say "0 grams" trans fat mean "less than half a gram", not necessarily zero!
Fat not found Nature. The vast majority of c-c bonds in all fats are single, aka saturated (no room for more hydrogen side atoms). If double bonds (unsaturated) present & converted to single bonds by industrial hydrogenation lacking control: ½ cis, ½ trans. All living cells do cis- see google images ancel keys started & marketed the myth, 1953, that sat. Fats of natural origin unhealthy & helped drive this problem.