Soaps and detergents are not the same thing, although both are surfactants, or surface active agents, which basically means a washing compound that mixes with grease and water.
Soaps are made of materials found in nature. Detergents are synthetic (although some of the ingredients are natural); they were developed during World War II when oils to make soap were scarce. There is little doubt that soap is better for your health and the environment than detergents. Some detergents are very toxic to fish and wildlife.
Nonetheless, a big drawback of washing with soap is that the minerals in water react with those in soap, leaving an insoluble film. This can turn clothes grayish, and the film can leave a residue (such as is found on shower stalls, for example). Detergents react less to minerals in water and for all practical purposes are the product of choice for laundry, unless you have very soft water. Those of you with hard water—which has a high mineral content—already know about this, I am sure.
If you choose to wash your clothes with a detergent—or the dishes, or some of my recipes asking for a biodegradable soap or detergent—you can ensure the least possible damage to the environment by selecting the most biodegradable products. Health food stores have a number of brands of detergent that are made with renewable materials instead of petroleum-based ingredients, and with natural essential oil fragrance and no dyes. They also sell liquid vegetable-oil soaps called castile soap.