The big difference is that soaps are made from animal or vegetable fats, while detergents are made from petroleum products.
Natural soap is a more environmentally friendly choice, since soap is made from renewable resources and is less toxic to fish and wildlife since soap does not use the chemical builders (like phosphates) which detergents use and which are not very biodegradable.
The drawback of using real soap is that soap is more reactive to minerals, and it is this reaction that leads to soap scum—the harder your water, the worse the scum.
But I do love real soap, and I love the way it makes my skin feel.
There are also some good online sources. One special source is The Enterprising Kitchen, a nonprofit social enterprise located in Chicago which enables lower-income women who have been unemployed and underemployed to sustainable employment after 6-12 months.
In producing natural soaps and spa products, female workers receive intensive workforce preparation and skills development including paid employment, work and life skills training, individualized career planning, high school equivalency preparation, technology training, financial planning and a variety of other support services.
I first bought their product in bulk from Overstock.com, but they now successfully manage their own store from their own web site. Each product includes the mission statement on its label and is signed by the woman who packages it; I have given these soaps as gifts (along with my own hand-knitted cotton chenille washcloths).
When I’m feeling thrifty, I buy good soap in bulk from Amazon.com, such as Olivella Face and Body Soap, All-Natural 100% Virgin Olive Oil from Italy, 5.29-Ounce Bars (Pack of 12). For liquid soap. I look for a vegetable castile soap and try to buy it in bulk, such as the excellent Dr. Bronner - Pure-Castile Soap Peppermint, 1 gallon, which I dilute for liquid hand soap and also use to make my own shampoo.
A longevitiy tip: if you dry out bar soap, it lasts longer. I unwrap my fancy bar soaps as soon as I buy them and put them in my clothing drawers. They make my clothes smell great, and when I need a new bar of soap, they ar sufficiently dry so that they last a good, long while.