What's the difference between a hyaluronic acid serum and a moisturizer with HA?

A dedicated HA serum is a lightweight, water-based step that draws moisture into the skin and is layered under a moisturizer to seal it in. Several candidates here (the Vichy and Hada Labo creams, for example) are actually moisturizers or lotions containing HA rather than pure serums, so check the format if you specifically want a thin booster step versus an all-in-one hydrator.