Hair serum is a styling product that coats the surface of your hair. It's made with a base of silicone, a rubber-like substance that sits on top of your strands. Hair oil is oil applied to the hair as a cosmetic, conditioner, styling aid, restorative or tonic. Humans produce natural hair oil called sebum from glands around each follicle. A hair mask is a deep conditioning treatment that helps to heal damaged hair. They contain rich ingredients such as natural oils and lipids, in heavier concentrations than normal conditioners.