Moisturizers come in many forms: Creams, serums, facial oils, lotions and masks can have wonderfully hydrating properties. Which one you choose, depends mainly on your skin type, time of application, personal preferences and time of the year.
Oil and water do not mix together, so if we want to enrich a hydrating water solution with protecting oils, we need an emulsifier. Without emulsifiers water and oil would separate, resulting in uneven and likely unusable textures and poorer absorption of heavier ingredients such as plant oils.