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!
Trans fats. Cis and trans are organic chemistry terms for mirror images of the same molecule. Trans fats look the same but have different properties than the naturally occurring fats. In general i recommend avoiding consumption of trans fats.