So, where does soap come from?
Soap is created when fats or oils (fatty acids) are mixed with an alkali (metal), such as lye. A chemical reaction called saponification occurs and soap is the resulting product. A soap cleaning capacity is affected by hard water.
How is it different than a detergent?
Detergents use a synthetic (man-made) surfactant in place of the metal/fatty acid salt that is used in soap. Detergents react less to minerals in hard water. Most detergents are made with petroleum-based materials but there are may new detergents being made with renewable materials. Then, what is a surfactant? All soaps and detergents contain surfactants (surface active agents) as their active ingredient. The head of the surfactant acts a a wetting agent, lowering the surface tension and allowing penetration of water molecules. The tails forms an emulsion with any oily particles and encapsulates dirt particles, allowing them to be washed away.
You will sometimes see the term "surfactant" being used to mean "detergent". This can be confusing because as I have just explained, both soap and detergent are surfactants. So which one is better?
Soap is wonderful. It gets things clean, works on grease but depending on the application may not always be the best choice. Soap can leave behind a film on surfaces, so it probably won't be my first choice for dish soap. However, for my skin it is absolutely my first choice. My skin needs that extra moisturizing that you can only get from a well made artisan soap.
There are many new detergents on the market. Decyl polyglucose is a very mild detergent made from sugar that has been extracted from corn. But, it is not organic so you can be pretty sure that it was made from GMO corn. You will find this detergent in many "natural" body washes, hand soaps, kitchen soaps and laundry detergents. Sodium laureth sulfoacetate (SLSa), sodium lauryl sulfate (SLS), or sodium laureth sulfate are not as mild but still considered low hazard on EWG's Skin Deep website. You will find them in body washes, bubble bars, bubble baths and foaming bath bombs.
If a Hollywood style bubble bath is your thing, you will definitely be looking for a good detergent. Soap, even liquid soap does not produce long lasting bubbles.
I prefer a soap based laundry cleaner because it leave your clothes feeling softer, even when the are line dried. It also does not leave a residue that causes itching and skin irritation. I hope I have made you feel more knowledgeable about the difference between soaps and detergents and helps you make informed choices about the products that you are using.