Man made fat. Trans fats is a man made fat changing the fat molecule so it last. It is found in foods with hydrogenated fat or partially hydrogenated fat. It is solid at room temperature (ex crisco) and is found in processed foods like potato chips and baked goods. This is a very bad fat linked to heart disease. Avoid this by not eating processed food. Nature does not make this...Only man does.
Processed foods. 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!
Vegetable Oils. All vegetable oils are liquid at room temp because of 1 or more double c-c bonds. Yet vast majority of c-c bonds: all fats: single. If double bonds (unsaturated) are present & converted to single bonds, aka saturated (no room for more hydrogen), the final linkage may be cis- or trans-; lookup: google images: "cis-fat vs. Trans-fat" all living cells do cis- ; industrial lacking control: ½ trans.