To hydrate your skin, you simply add water, such as when you wash your face or take a bath. To moisturize your skin, you add water, oil, or both.
For most skin types, you should moisturize skin in the morning and evening. Use a moisturizer that does not aggravate a skin problem.
For instance, never put heavy cream on oily skin that's prone to breakouts. For oily skin, the moisturizer should be oil-free, but have hydrating qualities. Always use moisturizers after cleansing and hydrating your skin.