I have lots of friends and customers ask me what is a serum and how or when do I use it? A serum is different to a moisturizer because it is usually lighter and doesn't contain ingredients that are designed to seal in moisture, they don't create a barrier over the skin like moisturizers do. Because of this they can penetrate the skin more deeply and so are good at providing active ingredients that really treat the skin and deliver nutrients and hydration deeper. A little usually goes a long way as the active ingredients are less diluted.
Serums should be applied after cleansing and before moisturizer or sunscreen. They absorb quickly and then can be followed with a moisturizer if you want that additional layer to hold hydration in.
This page on healthline.com summarizes it all well:
I use to think that essential oil based serums would be oily and stay on top of the skin, but because of the quality of ingredients they absorb right in, and really feel like your skin is being nourished and nurtured - like a good nutritious meal instead of something that will quickly fill you up.
Find our serums here.