In the simplest of terms, soap is fat. Specifically, it is fat that is transformed. Any kind of fat can provide the foundation for this transformation. There are two kinds of fat: animal and vegetable. Most soaps utilize both animal and vegetable fat. However, some soaps, a minority, forego the animal fat and are made with pure vegetable fat. Let's define animal fat and vegetable fat.

When we speak of the animal fat that is used in soap, we are usually speaking about tallow. Tallow, as defined by American Heritage, refers to: "Hard fat obtained from parts of the bodies of cattle, sheep, or horses, and used in foodstuffs or to make candles, leather dressing, soap, and lubricants." When we speak of vegetable fat, the reference is to vegetable oil.