Lecithin

Lecithin is one of a class of biological compounds called phospholipids; in fact, it is actually a combination of phospholipids (Phosphatidylcholine, Phosphatidylethanolamine and Phosphatidylinositol) with a mixture of unsaturated fats.
Lecithin forms a very large part of the structure of cell walls and other membranes such as the working membranes inside mitochondria; as such, lecithin is essential to life. However, the body is capable of making its own lecithin from its components for itself, so lecithin is not a vitamin. Nevertheless, lecithin is still a very useful supplement because the unsaturated oils and the choline in it tend to be in short supply in the diet.
Lecithin is particularly useful for the health of the nervous system including the brain, because nervous tissue is very high in lecithin and the insulation around the axons (the "trunk line" nerves of the body) is almost entirely made from lecithin.
Lecithin is usually extracted, for commercial purposes, from soya beans. The richest conventional food source of lecithin, however, is egg yolks. This does not make egg yolks a very good source of lecithin for those wanting to use lecithin to reduce blood cholesterol, because the lecithin is combined in this case with large amounts of cholesterol, thus nullifying the cholesterol-reducing properties of the lecithin.
Lecithin is used extensively in commerce as an emulsifier, in order to stop oil and water-based products (such as salad cream) from separating. Lecithin's other main use is of course as a supplement.
Lecithin, apart from its effect on cell membrane integrity, is also an emulsifier. This means that lecithin mobilises fatty substances such as cholesterol, and can help reduce or eliminate cholesterol deposits on the walls of blood vessels.
Lecithin's main uses as a supplement, therefore, are to improve the health of the nervous system and brain and to reduce the buildup of cholesterol deposits in the arteries, which if unchecked can lead to various circulatory disorders such as heart disease.