Partially hydrogenated oils are the result of hydrogen atoms being added to fats. This process is call hydrogenation, and it has a dramatic impact on the physical properties of fats.
Saturated fats contain the maximum possible number of hydrogen atoms. The process of hydrogenation causes fats to become saturated.
The most common reason for hydrogenating an oil is to give it a longer shelf life, because it oxidizes less readily. It also has a thicker texture, which is useful in baking.
The addition of hydrogen atoms in an oil increases its melting point, which is what makes partially hydrogenated oils thicker than unhydrogenated oils.
Saturated fats can increase the level of LDL cholesterol in the blood. This type of cholesterol increases the risk of coronary heart disease.
Margarine is made by hydrogenating oil until it becomes a solid at room temperature.