Fat is the animal equivalent of a carbohydrate. Which is to say, it’s how animals store energy. You might remember from previous posts on frying that fat molecules are “E” shaped. They’re called triglycerides, which means they’re made up of three fatty acid molecules connected to a “backbone” of one glycerine molecule.
The kind of fat the trigyceride is is determined by what kinds of fatty acids are attached to the glycerine backbone. Some 40 different kinds of fatty acids exist in nature, which makes for an awful lot of variety. Both animals and some plants produce fats. In general, animal fats tend to be solid at room temperature (which is why we call them “fats”) and plant fats are liquid (which is why we call them “oils”).