Moisturizers are cosmetic products applied to the skin top add or restore moisture to it. They should add water to the skin and bind it there. Effective moisturizers consist of water, humectants, occlusives and emollients (read a detailed article here).
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. For example, in colder months or for dry and mature skin, richer creams can be the best choice. For young skin, a hydrating serum can be sufficient to keep hydration levels of the skin at the optimum level. Masks can be powerful hydration boosters as they are normally richer and are left on the skin for at least 15 minutes allowing the added moisture to be absorbed - so that your skin feels comfortable, supple and healthier.