Serum, Ampoule & Essence
Serums are skin care products that are designed to deliver high concentrations of specific active ingredients to the skin. Most often, serums are clear, gel-based or liquid, and they tend to be less thick than a moisturizer. They're typically applied before moisturizers to help lock in moisture.
An ampoule is a supercharged serum with higher concentrations of active ingredients that you're meant to use for a finite amount of time as a booster or when you're having a skin crisis of some sort.
Facial essence is a lightweight, hydrating liquid that is used to prepare the skin to absorb moisturizer and subsequent products better. It delivers active ingredients to the deepest layer of the pore which makes the skin ready to accept other products.