Chemically, soap is a salt of fatty acids. The domestic uses of soap include washing, bathing and other types of domestic services, in which soap acts as a surfactant and emulsifies oil so that it can be taken away by water. In industry, they are also used in textile spinning [need to be further explained], and are important components of some lubricants. Metal soap is also included as a rheological modifier in the modern artist's oil paint formulation.

Soap for cleaning is obtained by treating vegetable or animal fats with a strong base (such as sodium hydroxide or potassium hydroxide) in an aqueous solution. The oil consists of triglycerides; three fatty acid molecules are attached to one glycerol molecule. Alkaline solutions are often referred to as lyes (although the term "lye soap" almost specifically refers to soap made from sodium hydroxide) and can induce saponification.

In this reaction, triglyceride fat is first hydrolyzed to free fatty acid, and then the latter combines with alkali to form coarse soap: various soap salts, excessive fat or alkali, water and amalgam of Free Glycerin (glycerin). Glycerin, a useful by-product, can be retained in soap products as a softener or separated for other uses.

Soap is the key ingredient of most greases, usually calcium soap or lithium soap and mineral oil emulsion. Many other metal soaps are also useful, including aluminum, sodium, and their mixtures. Such soap is also used as a thickener to increase the viscosity of the oil. In ancient times, grease was made by adding lime to olive oil.

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