There are various types. Mainly from study of skin cell natural moisturising chemicals (NMFactor or NMF), such as
the more expensive hyaluronic acid or various ceramides. (Using hyaluronic acid as moisturiser is very common, but often misused. It is a humectant, namely it can draw water out from beneath the skin. However if you don't put another
oil-based film on top of it, after evaporation it will leave the skin even more dry, making its effect short lived).