Cetostearyl alcohol is a fatty alcohol that's a mixture of cetyl and stearyl alcohols. It's a white, waxy solid that's used in many cosmetics and personal care products: 
Emulsifier: Helps prevent products from separating
Opacifying agent: Contributes to the opacity of products
Foam booster: Helps create foam
Viscosity-increasing agent: Helps increase the viscosity of products
Skin and hair softener: Helps soften skin and hair
Conditioning agent: Helps condition hair and tackle frizziness
Soothes skin: Soothes the skin when applied to the scalp and hair