Serums and creams are frequently used interchangeably and are built to do different things. The distinction is in the base rather than the active ingredients, and it determines where each product ends up.
The base determines where the product sits
A serum is usually water-based or a light gel, with little oil and few thickeners, so it spreads thinly and absorbs quickly.
A cream is an emulsion of oil and water stabilised with emulsifiers and thickened to remain on the surface for longer.
That difference in residence time is the functional distinction. A serum delivers and clears; a cream stays and forms a layer.
Concentration is easier to achieve in a light base
Many water-soluble active ingredients are difficult to keep stable and evenly dispersed in a thick emulsion.
A simpler base with fewer competing components allows higher concentrations to be formulated and kept stable through the product's shelf life.
This is why actives are commonly presented as serums, and why the same ingredient in a cream may be present at a lower level.
Creams do work a serum cannot
Reducing water loss from the skin requires a film on the surface, and that requires occlusive and emollient materials that a light base does not carry.
Emollients also fill gaps between surface cells, which smooths texture immediately and is a large part of why a cream feels different on application.
A serum applied alone leaves nothing behind to perform either function, which is the reason for the conventional order of applying one and then the other.
Stability constraints differ between the formats
Emulsions can separate under temperature change or over time, so creams require emulsifier systems and testing that a single-phase serum does not.
Water-based products need robust preservation because water supports microbial growth, and packaging that limits contamination.
Anhydrous formats, containing no water at all, avoid the preservation problem but cannot carry water-soluble ingredients, which is a genuine trade-off rather than a shortcut.
Texture preference is a real constraint on formulation
A product that is technically excellent and unpleasant to apply will not be used consistently, and consistency is what most results depend on.
Formulators therefore spend considerable effort on sensory properties, including how a product spreads, how quickly it absorbs and what it leaves behind.
Choosing between the formats sensibly means matching them to skin behaviour and climate rather than assuming that a richer texture is always doing more.