Serums are everywhere nowadays, but what do they actually do?
They're meant to be an extra step in our beauty routines, for when skin is in need of a boost, rather than a replacement. So they should be used before your usual moisturiser. But because they come in liquid form, they do away with all the emollients and ingredients needed to make creams the right consistency and are therefore more powerful, with concentrated ingredients that act like a booster shot.
And because they're typically sold in push dispensers or dropper bottles, their active ingredients are also protected, which prevents them deteriorating before they hit the skin.