Serum is a vehicle for active ingredients. We use serums before moisturizer to add nutrients into deeper layers of your skin. It also improves the absorption of your moisturizer. A moisturizer has larger molecules than a serum. Serums deposit nutrients, moisturizers are meant to deposit moisture and hydration your skin. You can apply the serum after your toner and before your moisturizer. Depends on your skin type you can use anti-aging, brightening, acne preventive and other serums.