Here's why:

* Esters are formed by the reaction of an alcohol and an acid. They have the general structure R-COO-R', where R and R' are hydrocarbon chains.

* Lipids are a diverse group of molecules, but many, like fats and oils, are composed of triglycerides. Triglycerides are esters formed from glycerol (a three-carbon alcohol) and three fatty acid molecules.

So, while lipids are a broader category, the core structure of many common lipids (triglycerides) is essentially an ester.