The main distinguishing feature between a fat and an oil is that a fat is a solid at room temperature and an oil is a liquid. There is no real distinction between the two on a nutrition label - both are "fats". There are two components that make up fats that we have in our diet and it doesn't matter if the fat is plant-based or animal-based. There is one other fat type that is steroids and specifically in that category, cholesterol. When all these greasy, oily, fatty types are lumped together in one big category, we have lipids. "Lipids" is the all encompassing broader metabolic "fat" category. We will focus on regular fats and oils because those are macronutrients. Below is a category/subcategory breakdown
The majority of all fats in our diets are actually triglycerides. A "glyceride" is a derivative of glycerol which is a 3-carbon, three hydroxyl group alcohol (HOCH2CHOHCH2OH, it's propane with a hydroxyl group on each carbon). The derivative is just an attachment through the hydroxyl group and the compound that joins in a fat is a special type of carboxylic acid called a fatty acid. A fatty acid is really the same as a carboxylic acid except that all the carbons are inline to form a long hydrocarbon chain. I say long because the majority are between 12 and 28 carbons, but you do have some as short as just 4 carbons. We already learned in the polymer chapter about condensation reactions and that the plastic PET is made via ester links. Those are the same links that are in triglycerides. The "tri" part of the name just means that all three hydroxyl group positions are linked. And yes, there are such things as just monoglycerides and diglycerides - but triglycerides are the major player and king of the fat mountain. Below is a schematic of the making of a triglyceride.
Although there ARE systematic names for fatty acids, the majority of them have common names that are used instead. In the structure below, the fatty acid part is systematically known as octadecanoic acid which means 8+10 or 18 carbons in the formula. The inline formula is written as CH3(CH2)16COOH. The more common name is much shorter and is stearic acid. Below I've show 3 stearic acid molecules reacting with glycerol to make a triglyceride.
Notice how stearic acid has all single bonds in its carbon-chain. When carbons are in a chain with all single bonds and "full" of hydrogens in the other positions, you have a saturated hydrocarbon and in this case a saturated fat. Inserting a double bond somewhere in the chain causes a region of unsaturation. Double bonds = unsaturated regions. If there is only one double bond you have a monounsaturated fatty acid, after one, meaning two or more double bonds we call them polyunsaturated fatty acids. Below is a set of three 18-C fatty acids with 1, 2, and 3 double bonds - their common names are given.
A true "fat" that is a solid at room temperature has no double bonds - it is saturated. Most animal-based fats are like this - totally saturated. An "oil" is a liquid at room temperature and will always have at least one or more double bonds. As a matter of fact, the more double bonds an oil has, the lower its melting point becomes. So why do those double bonds lead to lower melting points?
A physical property such as melting point and others is always due to the magnitude of the intermolecular forces (IMFs) within that molecule. So turning this around we could instead ask the question - How does having a double bond mean LESS IMFs in oils that in a saturated fat? The answer is two-fold. First, because both fats and oils are hydrocarbons, this means they are non-polar. Being non-polar means that the dominant IMF holding molecules together are dispersion forces. Dispersion forces are based on temporary dipoles and are extremely weak for small molecules. But dispersion forces scale nicely with size of the molecule and, to be more specific, they scale with the surface area of the molecule. Get a large molecule that is "spread out" will increase its surface area and therefore increase the attractions due to dispersion forces. The molecule will also need to have a favorable shape so that close approaches can be made.
Now the second part of that two-fold answer. Those double bonds in oils always have a cis configuration and not trans. A cis configuration is when the carbon chains (or other comparable groups for that matter) that are on each end of the double bond are on the SAME SIDE of the double bond axis. A trans configuration is when you have groups on OPPOSITE SIDES of the double bond axis. This is shown for the two isomers of 2-butene below. The formula and connections are identical: a double bonded set of carbons with a methyl group on each one. However, their configurations are different because the double bond means no rotation about that bond (it's locked!) and the geometry is trigonal planar where all the carbons are in the same plane and the bond angles are 120°.
More naming trivia... In the IUPAC world of naming chemicals, cis and trans are replaced with the italized capital letters in parentheses (Z) and (E). (Z) is the cis equivalent and comes from the german word zusammen which means "together". (E) is the trans equivalent and comes from the german word entgegen which means "opposite". So the names above (according to IUPAC) are (Z)-but-2-ene and (E)-but-2-ene respectively. Yeah, the number 2 gets put inside the name butene to establish the position of the double bond. We will stick with our putting the number out front like is show in the figure.
Every double bond in those mono- and polyunsaturated fats (the oils) are cis in their configurations. This puts permanent bends or kinks in the nice long carbon chain and ultimately screws up the nice linear chain that can stack nicely and have significant dispersion forces. This wrinkle or kink means less IMFs and therefore lower melting points.
Why is that important? Well there IS a problem around the world - but really a big problem in this country - with cardiovascular disease. Those clogged arteries are clogged with squishy greasy solid fats - a lot of saturated fats which tend to make better solids than any oils. Learning something about structure and interaction in chemistry can lead to a better understanding of the chemistry/physics of your physiology and biochemistry. And ultimately lead to better drugs and better treatments to combat this killer disease.