Fats trans, saturated and otherwiseSaturated fat is another name for fully hydrogenated fat. This means that all the double bonds (two carbon atoms that are bonded together by two bonds, not just one, and sometimes written as C=C) in the fat have been converted to single carbon-carbon bonds by the addition of hydrogen. Found in butter, meats, some plant oils (like coconut and palm kernel), saturated fat has been associated with heart disease and artherosclerosis, though some critics say that studies may have confounded saturated fats with trans fats.
A fully or mostly unsaturated fat (like the polyunsaturated fat in olive oil or canola oil) has most of its carbon-carbon bonds as double bonds, but not in the trans orientation (the alternative orientation is called cis). Many studies suggest that replacing saturated fats in the diet with unsaturated fats will increase one's ratio of HDL (good) to LDL (bad) serum cholesterol.
Because the trans in trans fat refers to an orientation around the carbon-carbon double bond, a fully saturated fat with no double bonds cannot be a trans fat. For the most part, trans fats do not exist in nature and are man-made, which may be one reason why the body has not evolved to cope well with them.
Confusing, isn't it. This may help. If you take an unsaturated fat (its in the cis form) and add some hydrogen to it, you get a trans fat with fewer double bonds. If you take that trans fat and add more hydrogen (as much as it will take), you get a saturated fat with no double bonds.
The food industry converts unsaturated fats into trans fats because they are easier to handle (think Crisco vs. liquid oil) and less prone to spoilage, especially at high heat. (That's because rancidity develops mostly from adding oxygen to those double bonds, and there are fewer of them.)
Bottom line: mono or polyunsaturated fats = good (as long as you don't eat too many of them), trans fats = bad, saturated fats = the jury is still out, but best guess is not so good.