Commercial shampoos are generally chemical bases, not acids, because they are intended to clean hair without scorching, etching, or damaging the follicle and scalp. However, shampoos also usually contain detergent cleaners, and some contain wax or silicon, which coats the hair for protection but makes it harder to clean or color. The No Poo movement is based on cleaning hair without artificial or harshly chemical products.
One very common No Poo hair treatment is a half-and-half baking soda and water mixture, to remove oils and dirt, followed by a half-and-half water and vinegar mix, to counteract the baking soda and leave the hair at a slightly higher pH (measurement of acidity). The baking soda is also a base, so the vinegar -- which users report does not leave a vinegar smell as long as it is rinsed thoroughly -- reacts with it, helping to lift off some of the oil and leftover dirt while not harming the scalp with harsh soap or detergent. Usually, the vinegar is not applied until the hair is washed clean of baking soda. Hair follicles lie flatter and are shiner at a slightly higher pH -- about 5.5 -- and the diluted vinegar reaches that level.
pH balance is important both in the body and in the hair. If the hair is too acid or too base, it will damage easier and fall out faster. Commercial shampoos and conditioners are usually pH balanced, but the other additives in them can be damaging as well; the baking soda and vinegar approach is a viable alternative for some people.