Fat is made up of glycerol and fatty acids. Specifically, a fat is composed of three fatty acids chains—composed of 16 or 18 carbon atoms in length—connected to a glycerol by way of an ester linkage.

Glycerol is composed of an alcohol attached to three carbons each bearing a hydroxyl group.

A saturated fatty acid has no double bonds between the carbon atoms making up the tail of the fatty acid. This lack of double bonding allows for as many hydrogen atoms as possible to be bonded to the carbon skeleton, thus the fatty acid is said to be saturated with hydrogen atoms.

An unsaturated fatty acid has one or more double bonds in its hydrocarbon tail. These double bonds are formed by the removal of hydrogen atoms from the carbon tail, resulting in a kink in the tail. As such, they are not fully saturated with hydrogen atoms. Cholesterol is a type of lipid that contains four hydrocarbon rings in its structure. Hydrogenated fatty acids are unnatural saturated fatty acids that are modified for their physical properties.   

A triacylglycerol (also called commonly referred to as a fat or a triglyceride) is composed of three fatty acid chains linked to glycerol via an ester linkage. The fatty acids can be the same or two or three different types. Steroids are derived from cholesterol, a molecule that contains four hydrocarbon rings attached together. Phospholipids only have two fatty acid tails linked to a glycerol head and a phosphate group.  

Triacylglycerol (also called a fat or triglyceride) is a type of lipid. Amino acids are the subunits that make up proteins. Chitin and cellulose are polysaccharides, or carbohydrates.

Structurally speaking, a triglyceride molecule contains three fatty acid chains covalently bonded to glycerol—specifically where the hydroxyl groups are located on the glycerol molecule.

The term saturated when describing a lipid means that a molecule has the maximum number of hydrogens on its carbons, it is saturated with hydrogens. An unsaturated lipid has less than the maximum amount of hydrogens bonded to its carbons because there is a double bond between two of the carbons. Lipids molecules can have one carbon to carbon double bond (mono-unsaturated) or multiple (poly-unsaturated).

Fatty acids occur as free molecules in trace amounts in living organisms. They usually occur as a component of several types of lipid molecules, and are not true monomers, even though 1-3 of them can be attached to a polar head group such as glycerol or a phosphate group. The rest of the choices listed form polymers. Amino acids form proteins, sugars such as glucose form carbohydrates, and nucleotides form both DNA and RNA.

Lipids can be used as an energy source and are often used by cells to store energy for future use. Phosphate groups are not found in all lipids. For example most steroids, which are lipids, do not have phosphate groups. Lipids are hydrophobic and are repelled by water.