OK, by now, you’ve probably heard about both — hydrating and moisturizing ingredients. The difference is that while hydrating ingredients add water to the skin, moisturizing ingredients prevent trans-epidermal water loss (TEWL) from the skin. In layman’s terms, you can hydrate your skin by giving it water or by preventing its evaporation from the skin. While humectants are the only hydrating ingredients that add water to the skin, moisturizers include occlusives and emollients, which cover the skin with a protective film to trap moisture in.