Phospholipids are a class of lipids and are a major component of all biological membranes.

All phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline. They are a type of molecule. They form a lipid bilayer within a cell membrane. They also transport DNA.

The 'head' of a phospholipid is hydrophilic (attracted to water) wheras the hydrophobic 'tails' repel water. When placed in water, phospholipids form a variety of structures depending on the specific properties of the phospholipid. In biological systems, the phospholipids often occur with other molecules (e.g., proteins, glycolipids, cholesterol) in a bilayer such as a cell membrane.

References

J.M.Berg, J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. viii, 974, [976] (various pages)