Lipid is a loosely defined term for substances of biological origin that are soluble in nonpolar solvents. They consist of saponifiable lipids, such as glycerides (fats and oils) and phospholipids, as well as nonsaponifiable lipids, principally steroids. Phospholipids are lipids containing phosphoric acid as mono- or di-esters, including phosphatidic acids and Phosphoglycerides. Phosphatidic acids are derivatives of glycerol in which one hydroxy group, commonly but not necessarily primary, is esterified with phosphoric acid and the other two are esterified with fatty acids. Phosphoglycerides are phosphoric diesters, esters of phosphatidic acids, generally having a polar head group (OH or NH2) on the esterified alcohol which typically is 2-aminoethanol, choline, glycerol, inositol and serine. Glycerophospholipids are any derivatives of glycerophosphoric acid that contain at least one O-acyl, or O-alkyl, or O-(1-alkenyl) group attached to the glycerol residue. (Source: IUPAC Gold Book)
Phospholipids have many important biological roles for living creatures. In this regard, phospholipids are famous for their capability of creating bilayer structure of cell membranes. Polar or ionic groups in the nearby of phosphorus atom can form a small polar and hydrophilic (literally water loving) head for the phospholipid molecule while the long chains of esterified fatty acids can be considered as a long non-polar and hydrophobic (lipophilic or literally fat loving) tail. Such amphiphilic nature (having both hydrophilic and hydrophobic parts) favors a bilayer structure in which hydrophilic heads of molecules in each layer are located toward outside while hydrophobic tails are located toward inside since in this way, favorite interactions, interactions of hydrophilic heads with surrounding aqueous media and interactions of hydrophobic tails with each other, are maximized while least-liked interactions of hydrophobic tails with polar heads and surrounding aqueous media are minimized. Such bilayer membrane will act as a fatty barrier between interior and exterior of cell and the cell can use such barrier to control input and output between the cell and its surrounding which is essential for the life of cell. In addition to formation of cell membranes, phospholipids also have other biological functions like signal induction and transport.