What is the difference between plaster, plasterboard, skim and render?

They are different materials and processes for finishing walls, and the terms are used interchangeably in a way that causes confusion when getting quotes.

Plaster is the material — a gypsum-based mix applied wet and set hard. Traditional solid plastering is applied directly to masonry in two coats: a browning or bonding undercoat building thickness and flatness, then a finish coat for smoothness. Older properties may have lime plaster, which is different in behaviour and important to preserve in period and listed buildings, because it allows walls to breathe.

Plasterboard (drywall) is factory-made sheets of gypsum core faced with paper. Rather than building thickness in wet plaster, you fix boards to create a flat surface quickly.

Two ways of fixing it:

Dot and dab — adhesive blobs bonding boards to masonry. Fast, and creates a void behind, which affects fixing and sound.

On timber or metal studs, for partitions and for lining walls.

Skimming is the process of applying a thin finish coat — typically 2–3mm of finish plaster — over plasterboard or existing sound plaster to produce a smooth paintable surface. "Skim" refers to that final coat, not to a material in itself.

Plasterboard can also be taped and jointed instead of skimmed, filling only the joints and screw heads. Cheaper and faster, common in commercial work, but the finish differs and it is less forgiving of poor board fixing.

Render is the external equivalent — a cement, lime or polymer-based coat applied to outside walls for weather protection and appearance. Types include traditional sand-and-cement, lime render for older solid-wall buildings, monocouche (a one-coat coloured through render), and silicone or acrylic renders over insulation.

Why the distinction matters practically: using cement render or gypsum plaster on a solid-wall period property can trap moisture and cause damp, which is why lime is specified for those buildings.