Trans fats raise LDL "bad" cholesterol and make you more likely to get heart disease. They also lower HDL "good" cholesterol.
Based on the a review of the scientific evidence, the FDA no longer recognizes trans fat that comes from partially hydrogenated oils as GRAS (generally recognized as safe). The ban doesn't affect the small amounts of natural trans fats found in beef, lamb, and full-fat dairy products. Companies have three years (from 2015) to phase out artificial trans fats from their products.
Food makers once used artificial trans fats to enhance the flavor, texture, and shelf life of processed foods. You might have seen those trans fats listed as "partially hydrogenated oils."