Fats and oils are terms used interchangeably. When it is liquid, it is called oil but when it is solid, it is fat.
Biochemically, fats and oils have a backbone of carbon atoms of varying lengths. Long chain fats like in animal fats have chains of up to 24 carbon atoms.
Each carbon atom in the molecule can form four bonds. When all bonds are single, the fat is saturated. When double bonds exist, the oil is unsaturated. By convention, the polyunsaturated fats like Omega 3, 6, 9 are named depending on the location of the first double bond.
The double bonds in the polyunsaturated fats and oils allow the molecules to bend and fold. Thus, these molecules have unique three-dimensional configurations which are absolutely critical for our cells to function properly.
These polyunsaturated fats and oils are called essential fatty acids for two reasons. The first is that our body is not able to produce these. They have to come from our diet. The second, more important reason is that they are need for the proper functioning of our cells.