Phospholipids are modified diglycerides and are usually found as an important component in cell membranes.
In phospholipids a molecule of phosphoric acid (a phosphate group) replaces the third fatty acid molecule in a standard triglyceride molecule creating a diglyceride molecule that is no longer neutral (it carries a negative charge). The fatty acid parts of the molecule may be saturated or unsatruated.
The phosphate group is then further modified by being joined to either a molecule of ethanolamine, choline or serine.
This complex structure is amphiphilic (parts of the molecule are hydrophilic and parts of the molecule are hydrophobic), which means that, in water, the molecules self assemble into double layers (bilayers) in which the long, hydrocarbon "tails" of the phospholipid molecule pack together in the center of the layer (away from the water) and the ionic and hydrophilic "heads" of the molecule interact with the water on either side of the layer.
The interior of the 30 Angstrom interior part of the bilayer is composed of a massive tangle of hydrocarbon chains which strongly repel water and are highly nonpolar.
Unlike soap molecules, which have similar properties, the phospholipids do not readily cluster together to form micelles (small spheres) but self-organize into the opposite facing layers; the planar bilayer, which is the major organizing structure in the typical cell membrane.