A cleanser is used to get out build up from products like hair spray, gel or wax to name a few. It can also be used to get out chlorine build up.
A cleanser can come as a shampoo or on its own. A cleanser is very useful when hair looks dull from build up from different styling products like hair spry wax and alike. Most people who use a chlorinated pool will have build up in the hair of chlorine, its nor so easy to find a product that will remove this as most products who claim to remove chlorine don't. There are some who do, may take some trying out of products before getting the one that works.
Cleasing hair tip: Mixing baking soda and shampoo in your hair once a week it will not only remove all build up products like wax, styling gels, and other products, it will remove impurities from the water and brighten your hair.