Without getting into a long scientific discussion about the chemical properties of soap vs. detergent, there are a few points I'd like to make about soap.
True soap is created when two natural ingredients – fat and lye – are combined, causing a chemical reaction called saponification. Other ingredients may be added to enhance the properties of the finished soap, but the base ingredients of all true soaps are fat and lye.
Believe it or not, most commercially available "soaps" are not true soap at all, nor do they claim to be. Notice that most of the products in the soap aisle are called "body bars", "cleansing bars" or "washes". So, if they aren't soap, what are they? They are detergents. Detergents are manufactured with synthetic ingredients, although some may contain a few natural additives. In addition to the chemicals necessary to give the detergent the ability to remove dirt, other chemicals are added to enhance the finished product. For example, a rich lather, a long-lasting bar, or "no-tears" formulas are all due to chemical additives.
Most people are so accustomed to using these chemical-laden products that they are surprised when a true natural soap doesn't lather quite as much, or doesn't last as long, and think that the true soap is an inferior product. Nothing could be further from the truth!
Both soap and detergent have their place and purpose in our lives, but when it comes to human skin, soap - especially handmade soap - is the far better choice. Handmade soap contains glycerin, the natural by-product of the saponification process, which is a humectant and is great for your skin. Commercial soapmakers remove the glycerin for resale, making commercial soap drying to the skin. The base ingredients of handmade soap include a wide variety of specialty oils, each of which imparts their own beneficial properties to the finished product. The lack of harsh chemical detergents in handmade soap makes it especially ideal for use on sensitive skin, baby skin, and mature skin. It is also biodegradable and better for the environment.
Now on to the methods used to make true soap.