Hydrogenation is the process of liquid fats such as sunflower, safflower, or soybean oil being hardened via the process of hydrogenation (heating oil and passing hydrogen bubbles through it). The fat that is formed as a result is called trans fatty acids. The fatty acids in the oil then acquire some of the hydrogen, which makes it denser. If you fully hydrogenate, you create a solid (a fat) out of oil. But if you stop part way, you have semi-solid partially hydrogenated oil that has a consistency like butter. This process alters the structure of some of the polyunsaturated fatty acids so that their effect on blood cholesterol is similar to that of saturated fatty acids.
Because of the consistency, and because it is cheaper to produce than butter, hydrogenated oil is a big favorite as a butter substitute among "food" producers. It gives their products a richer flavor and texture, but does not cost as much as adding actual butter.
Hydrogenation makes a fat more saturated. It is thought that the use of trans fats is a more significant contributor to heart disease that the use of saturated fats. The best choice is margarine over butter, and the best margarine choices are those that list a liquid oil as the first ingredient, some margarine are now trans fat free. These products will be less saturated. Palm and coconut oils, as well as animal fats and butter, are high in saturated fats.
Dietary recommendations for the maximum percentage of daily calories from fat are: total fat calories should be no more than 30%, with the bulk of these calories coming from unsaturated fats and not more than 10 percent of total daily calories from saturated fats.