Any of a diverse group of organic compounds, occurring in living organisms, that are insoluble in water but soluble in organic solvents, such as chloroform, benzene, etc. Lipids are broadly classified into two categories: complex lipids, which are esters of long-chain fatty acids and include the glycerides (which constitute the fats and oils of animals and plants), glycolipids, phospholipids, and waxes; and simple lipids, which do not contain fatty acids and include the steroids and terpenes.

Lipids have a variety of functions in living organisms. Fats and oils are a convenient and concentrated means of storing food energy in plants and animals. Phospholipids and sterols, such as cholesterol, are major components of plasma membranes (see lipid bilayer). Waxes provide vital waterproofing for body surfaces. Terpenes include vitamins A, E, and K, and phytol (a component of chlorophyll) and occur in essential oils, such as menthol and camphor. Steroids include the adrenal hormones, sex hormones, and bile acids.

Lipids can combine with proteins to form lipoproteins (e.g. in cell membranes). In bacterial cell walls, lipids may associate with polysaccharides to form lipopolysaccharides.

http://www.lipidlibrary.aocs.org/ Extensive information about lipids in Lipid Library, compiled by the former head of the Scottish Crop Research Institute