Moisturizer is a single word covering three different actions. Products that feel similar can be doing quite different things to the skin surface.
Humectants draw water inward
Humectants are water-attracting molecules — glycerin, hyaluronic acid, urea and others — that bind water and hold it in the upper skin layers.
The water they attract comes from deeper skin layers and, in humid conditions, from the surrounding air. Direction of movement depends on the humidity gradient.
In very dry air a humectant applied without a sealing layer can draw moisture from the skin toward the surface where it evaporates, which is why layering matters.
Occlusives seal water in
Occlusives form a hydrophobic film that slows evaporation from the skin surface, a process measured in dermatology as transepidermal water loss.
Petrolatum is the most effective of these and is the reference against which others are compared. Its drawback is texture rather than performance.
Occlusives add no water themselves. They preserve what is present, which is why they are typically applied over damp skin rather than dry.
Emollients change how the surface behaves
Emollients are lipids that settle between the flaking cells of a dry surface, filling gaps and producing the immediate smoothness people associate with a good product.
Some also contribute to the lipid matrix between cells — ceramides, cholesterol and fatty acids are the components the barrier itself is built from.
This is the sense in which certain products are described as supporting barrier repair, supplying materials the skin uses rather than merely coating it.
Why formulations combine all three
Each mechanism alone is incomplete. Water attracted without a seal evaporates; a seal without water preserves dryness; smoothing without either is temporary.
Commercial products therefore blend the categories, and the differences between a light lotion and a heavy ointment are largely differences in proportion and water content.
Texture follows from that ratio, which is why heavier products generally perform better on very dry skin and worse on skin prone to congestion.
What application timing changes
Skin absorbs water during bathing, and the minutes afterward are when evaporation is fastest as that water leaves.
Applying a product while skin is still damp captures water already present rather than relying on the product to supply it.
Persistent dryness, cracking, itching or inflammation may indicate a condition requiring treatment rather than a moisturizer, and warrants assessment by a physician or dermatologist.