Lipids:

Triglycerides are formed by the condensation reactions of one molecule of glycerol and 3 molecules of fatty acids to create ester bonds:

Triglycerides have fatty acids with double bonds which increase solubility. Triglycerides that are liquids at room temperatures are classed as oils and any that are solids are classed as fats. Phospholipids are used in the bilayer of cell membranes where it is impermeable (only allows water and gases to diffuse in or out and not large molecules) as it has a hydrophilic head and hydrophobic tails: