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Lipids are a diverse group of organic molecules that have one main characteristic: they are hydrophobic (water-insoluble) due to their nonpolar structure. These include fats, oils, waxes, and steroids. The most common lipids are triglycerides, which consist of a glycerol molecule (a three-carbon alcohol) bonded to three fatty acid chains through ester bonds. Fatty acid chains are long hydrocarbon chains with a carboxyl group at one end, which can be saturated (full of hydrogen atoms, all single carbon-carbon bonds) or unsaturated (some carbon-carbon double bonds) depending on the lipid.

Phospholipids are a subgroup of lipids that have a unique structure, consisting of a glycerol molecule, two fatty acid chains, and a phosphate group. The phosphate group is typically attached to the glycerol molecule by a phosphodiester bond, replacing one of the fatty acids found in a triglyceride. This phosphate group usually carries a negative charge, and it can also be attached to another charged or polar molecule, which we'll call the "headgroup". Due to the presence of the phosphate group and headgroup, the phospholipid has both a hydrophilic (water-loving) polar head and hydrophobic (water-insoluble) nonpolar tails.

The unique structure of phospholipids, with their polar head and nonpolar tails, allows them to form a bilayer in water, a key component of cell membranes. When placed in water, the polar (hydrophilic) heads of the phospholipids interact with the aqueous environment, and the hydrophobic tails aggregate away from the water molecules. Therefore, they naturally organize themselves into a bilayer, with the hydrophilic heads facing the aqueous environment on both sides and the hydrophobic tails sandwiched in the middle, away from water. This arrangement is driven by the process of hydrophobic interactions and is thermodynamically favored. The bilayer formation provides a stable barrier between the inside and outside of the cell, while also permitting selective transport of molecules across the membrane.