fatty acid and triglyceride
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When people talk about the fat in food or the fat stored on the body, the molecule they mean is the triglyceride, and its main ingredient is the fatty acid. These are the cell's dense, long-term energy store — the equivalent of a battery packed away for a rainy day.
A fatty acid is a long chain of carbon and hydrogen (a hydrophobic tail) capped at one end by an acidic carboxyl group. A triglyceride is three of these fatty acid tails attached to a small backbone molecule called glycerol — built, like all such bonds, by dehydration synthesis that removes water at each join. If the carbon tails carry no double bonds they pack tightly and the fat is solid at room temperature (saturated, like butter); if they have double bonds that kink the chain, they cannot pack as well and stay liquid (unsaturated, like olive oil).
Triglycerides matter because they store far more energy per gram than carbohydrate, and because they are so water-fearing they can be packed away in droplets without dragging along heavy water. That is why animals store long-term reserves as fat rather than sugar. The same chemistry explains a kitchen fact and a health one: saturated fats are solid and stable, unsaturated fats are liquid and more prone to going rancid.
Butter is solid at room temperature because its fatty acids are saturated and pack tightly; olive oil stays liquid because its unsaturated fatty acids are kinked and cannot.
Saturated versus unsaturated: a single kink in the tail decides solid or liquid.
Fatty acid is the building block; triglyceride is the storage form. Calling stored body fat fatty acids is loose — the body banks it as triglyceride and releases free fatty acids when it needs fuel.