Interestrified fat is a type of oil in which the fatty acids have been rearranged. This process changes the melting point, lengthens shelf life, and makes a more suitable and lower saturated fat margarine or deep-frying oil. Interestrified oil does not contain trans fat as does partially-hydrogenated oil, though one study showed that it may affect production of insulin and may cause drop in blood pressure. Interestrified fats are heavily processed and their molecular structure is not found in natural fats.
Science (simple overview)
All fats are primarily made up of triglycerides, which contain three fatty acid molecules. These triglycerides contain saturated fat, monounsaturated fat, and polyunsaturated fat. Generally, unsaturated fat tryclicerides are created and then blended with unaltered liquid oil, which makes the oil better for use in food and lower in saturated fat.