Trans fat is the common name for a type of unsaturated fat with trans-isomer fatty acid(s). Trans fats may be monounsaturated or polyunsaturated but never saturated.
Unsaturated fat is a fat molecule, containing one or more double bonds between the carbon atoms. Since the carbons are double-bonded to each other, there are fewer bonds available for hydrogen, so there are fewer hydrogen atoms, hence "unsaturated". Cis and trans are terms that refer to the arrangement of chains of carbon atoms across the double bond. In the cis arrangement, the chains are on the same side of the double bond, resulting in a kinked geometry. In the trans arrangement, the chains are on opposite sides of the double bond, and the chain is straight overall.
The process of hydrogenation is intended to add hydrogen atoms to cis-unsaturated fats, eliminating a double bond and making them more saturated. These saturated fats have a higher melting point, which makes them attractive for baking and extends their shelf-life. However, the process frequently has a side effect that turns some cis-isomers into trans-unsaturated fats instead of hydrogenating them completely.
There is another class of trans fats, vaccenic acid, which occurs naturally in trace amounts in meat and dairy products from ruminants.
Unlike other dietary fats, trans fats are not essential, and they do not promote good health. The consumption of trans fats increases one's risk of coronary heart disease by raising levels of "bad" LDL cholesterol and lowering levels of "good" HDL cholesterol. Health authorities worldwide recommend that consumption of trans fat be reduced to trace amounts. Trans fats from partially hydrogenated oils are more harmful than naturally occurring oils.