Trans fatty acids are mostly produced industrially by modifying the components of vegetable oils during a process called partial hydrogenation (a chemical process that involves turning unsaturated fats into partially or fully saturated fats). Trans fatty acids are also found in small amounts in animal products such as meat and butter.
They are considered the worst type of fat you can eat. They both raise bad cholesterol and lower good cholesterol. They should not account for more than 1% of the daily energy intake.
Fortunately, improvements in the hydrogenation process and the use of different ingredients are reducing the amount of trans fatty acids in food.