Fat is one of the three main classes of food. It has many meanings, and the only good description is in technical terms. There are triglycerides, fatty acids, sterols, but "fat" may also refer to adipose tissue.
The main constituents of both animal fat and vegetable oils are triglycerides, that is glycerin molecules with three fatty acids attached. Nutritional fat is a mixture of many types of triglycerides, which differ in the length of their fatty acid chains, as well as the number and position of single and double bonds between their carbon atoms.