Back up to the early 1900's. This is when trans fats were first discovered. Scientists found that by pumping hydrogen in liquid vegetable oil, a process called "hydrogenation", that they could turn a liquid fat into a solid fat.
Why were they concerned with doing this, you may ask? There are many reasons. For one, they extend the shelf life of foods. This is important to food manufactures today, but was also important during times of war, when shipping foods to troops. American households love it for other reasons, however. Margarine (high in trans-fat) stays soft when refrigerated, making it easy to store, then grab to spread on toast, or work into a batter. Butter goes rancid if left on the counter too long, and is too hard to work with when stored in the refrigerator. Margarine is also much cheaper than butter. These are just a few reasons why trans fats entered the market and were so well accepted. But, in the 1950's, research began discovering the negative health effects that came with trans-fat consumption.
In 2006, the FDA mandated that trans-fat be included on the nutrition label of all foods. If less than 0.5g per serving, however, it does not need to be labeled. So, trans fats could still be present in your foods and not labeled. How then, can you identify if trans fats are found in your food? Look at the ingredient deck. If you find the words "partially hydrogenated" in the label, you know it contains trans fats. If "fully hydrogenated" or "completely hydrogenated" oils are listed in the ingredient deck, they do not include trans fats. You may find some labels that say "hydrogenated oil", and in this case, you can probably assume that there is some trans-fat in the product.