Lipids are a diverse group of biological compounds, but they share the general property of being at least partially not water soluble and are composed of a series of hydrocarbon chains, ending with groups of bonded hydrogen and oxygen. Types of lipids include oils, fats, waxes, phospholipids and steroids. Lipids are crucial to all forms of life and are the main constituent of every cell membrane on Earth. They are also a major form of energy storage for animals and other organisms.
All lipids are composed of a number of basic units known as fatty acids. These fatty acids are hydrocarbon chains with oxygen-hydrogen ends, making individual fatty acids hydrophobic at one end and hydrophilic at the other. This is why soaps made by processing normal lipids both dissolve in water and emulsify lipids. Cell membranes are made from phospholipid bilayers — two layers of lipids with hydrophobic ends facing the cell's exterior and interior — but hydrophilic ends facing each other within the membrane.
Most lipids, however, are structured so that any polar hydrophilic areas are hidden and do not dissolve in water at all. The only consistent difference between fats and oils, both types of triglycerides consisting of glycerol bonded to three fatty acid chains, is melting temperature.