There are four kinds of fats: monounsaturated fat, polyunsaturated fat, saturated fat, and trans fat. Monounsaturated fat and polyunsaturated fat are the "good" fats. It is generally accepted that consumption of saturated fat should be kept low, especially for adults. Trans fat (which means trans fatty acids) is the worst kind of fat, far worse than saturated fat.

Trans fat is created by the partial hydrogenation of vegetable oils. Hydrogenation is the process of heating an oil and passing hydrogen bubbles through it. The fatty acids in the oil then acquire some of the hydrogen, which makes it more dense. If you fully hydrogenate, you create a solid (a fat) out of the oil. But if you stop part way, you have a semi-solid partially hydrogenated oil that has a consistency like butter, only it's a lot cheaper.
Partially hydrogenated oils are commonly found in processed foods like commercial baked products such as cookies, cakes and crackers, and even in bread. They are also used as cooking oils (called "liquid shortening") for frying in restaurants

The problem with trans fats is that while the "business end" (the chemically active part) is messed up, the "anchor end" (the part that is attached to the cell wall) is unchanged. So they take up their position in the cell wall, like a guard on the fortress wall. But like a bad guard, they don't do their job! They let foreign invaders pass unchallenged, and they stop supplies at the gates instead of letting them in.
In short, trans fats are poisons, just like arsenic or cyanide. They interfere with the metabolic processes of life by taking the place of a natural substance that performs a critical function. And that is the definition of a poison. Your body has no defense against them, because they never even existed in our two billion years of evolution -- so we've never had the need or the opportunity to evolve a defense against them.

But the worst part is that in the last stages of oil processing (or "refining"), the oil is literally steam distilled to remove its odor. So it doesn't smell. But a hydrogenated oil is much worse than rancid butter. So if it did smell, it would smell worse than the most rancid butter you've ever seen. (And that goes for all refined oils, not just the hydrogenated ones. It's just that hydrogenated oils are everywhere in the American diet.) So the next time you see "partially hydrogenated oil" on a label, think "rancid butter".
Partially hydrogenated oils will not only kill you in the long term by producing diseases like multiple sclerosis and allergies that lead to arthritis, but in the meantime they will make you fat! The consumption of trans-fats can also be a significant contributing factor in the development of depression, according to a recent study.
Trans-fats can raise LDL cholesterol and increase the risk of heart disease while also lowering beneficial HDL cholesterol.

Worse, most partially hydrogenated oil is partially hydrogenated soybean oil. That's a problem, because soybean oil depresses the thyroid--which lowers your energy levels, makes you feel less like exercising, and generally makes you fatter!

The percentages you see on food labels are not percentages of the food, but rather the percentage that the food's "Standard Serving Size" represents in a 2,000 calorie diet.
So manufacturers can play with the standard serving size (making it half a candy bar for example), and anything less than 10 trans fat calories per serving is less than .5% of a 2,000 calorie diet. The law allows a company to claim 0% in that case. So "0% trans fat" does not mean "no trans fat". It means that, at 9 calories per gram, the "standard serving" could still contain slightly more than 1 gram of trans fat (1 gram @ 9 calories per gram = 9 calories).
Since partially hydrogenated oils are 50% trans fat, a "standard serving" could have as much of 2 grams of partially hydrogenated oils, and still claim "0% trans fat". Clearly, that kind of labeling is of no value whatever to the conscientious consumer.

You need to read food labels and avoid anything that contains the words "hydrogenated". That means partially hydrogenated oils, hydrogenated oils, or anything of that kind (and mono-diglycerides, as well).

WHAT CAN YO DO?

When eating out, avoid deep-fried foods at all costs.

When you see a food that contains partially hydrogenated oils (especially if it claims to be healthy), put it back on the shelf.