Definition of a lipid
Lipids are organic compounds containing mainly carbon and hydrogen with subsidiary oxygen, nitrogen, phosphorous, and sulfur. Lipids are made by living organisms and are operationally defined as being insoluble or immiscible in water, and soluble in non-polar solvents such as chloroform. Lipids comprise a tremendous array of chemical structures and functions. Most importantly, lipids are the building blocks of organismic cell membranes, that is, the stuff that separates the cell’s self from its non-self.
Following is a rough map as to how to think about lipid classification.