Lipids are fats and oils. One lipid molecule is made from one glycerol molecule joined to three fatty acids. Making a lipid in this way is synthesis. Unlike starch and protein, a lipid is not a long chain of one repeating monomer, but fatty acids and glycerol are the building blocks.
Lipids are useful to the body. They are an energy store, they act as insulation to keep the body warm, and they are part of the cell membrane.
Lipase enzymes break lipids down into fatty acids and glycerol during digestion. These small molecules are absorbed, and cells can use them or rebuild them into lipids. To test for lipids, shake the sample with ethanol, then pour the mixture into water. A cloudy white layer, an emulsion, shows a lipid is present.