A class of Membrane Lipids that have a polar head and two nonpolar tails. They are composed of one molecule of the long-chain amino alcohol Sphingosine (4-Sphingenine) or one of its derivatives, one molecule of a long-chain acid, a polar head alcohol and sometimes phosphoric acid in diester linkage at the polar head group. (Lehninger et al, Principles of Biochemistry, 2nd ed)