Phospholipids are diglycerides that are covalently bonded to a phosphate group by an ester linkage.
The diglyceride is composed of a glycerol backbone that has esterified to two fatty acids . In many cells the phospholipids are further modified by the covalent bonding of additional compounds the the phosphate. Regardless of the exact structure of the phospholipids, cell membranes are composed of two layers of phospholipids in what is terms a bilayer arrangement. Thus cell membranes are, in part, phospholipid bilayers.