Oils that have been hydrogenated to be artificially "hard" at room temperature. Hydrogenated oils are found in just about any processed foods at the grocery store. They are found in all fast food chains, used to make french fries, doughnuts, and fried goods such as chicken. Hydrogenated oils are high in saturated fat (hydrogenated means artificially saturated) and are highly deficient in essential fatty acids like linolenic and linoleic acid, because these highly unsaturated fatty acids tend to deform into other fats and plastic-like trans isomers under high pressure and heat. Hydrogenated oils are associated with problems like insulin resistance, lipid peroxidation, increased LDL cholesterol with decreased HDL, and irritable bowel syndrome.
Hydrogenated oils are nearly impossible to avoid without preparing your food yourself. Just about all margarines contain partially hydrogenated oils, which contain molecular anomalies known as "trans fatty acids," which have no legitimate use in the human body. Unfortunately they can be shoved into cell membranes, and they literally have a hardening effect on your very cells.
In my own personal diet, I have noticed a correlation between trans fat consumption and irritable bowel syndrome / ulcerative colitis symptoms. In general, I eliminate them from my diet, but if I end up going to a restaurant (where they are almost completely unavoidable), I generally suffer from mild but quite noticeable <plug>rectal bleeding</plug> the next day.