Lipids are also known as fats, but they include compounds of many different kinds. Chemically, these compounds are built on a backbone of glycerol, which was a three-carbon chain. Each carbon, in turn, bonds via an oxygen molecule to so-called fatty acids. Fatty acids have an acid group (COOH) at one end that is attached to a chain of carbons. Lipids differ mainly in the nature of the fatty acid chains.