Jewelry described as gold that costs very little is usually a layer of gold over something else. The terms used for those layers are defined rather than decorative, and the differences are substantial.
Plating is a microscopically thin deposit
Electroplating passes current through a solution to deposit a film of gold onto a base metal item. The layer can be extremely thin, measured in fractions of a micron.
At that thickness the gold provides color and little else. Ordinary wear at contact points will eventually expose the base metal underneath.
Because so little metal is present, the gold content contributes almost nothing to the item's value. What you are buying is finish, not material.
Gold filled is a mechanically bonded sheet
Gold filled construction bonds a sheet of karat gold to a base metal core under heat and pressure, producing a layer orders of magnitude thicker than plating.
American labeling convention expresses this as a fraction of the item's total weight, which is why gold filled stock is described using a weight ratio alongside a karat.
The practical consequence is durability. A gold filled piece can be worn for many years before the surface wears through, which plating rarely survives.
Vermeil sits between the two
Vermeil is gold plating applied over sterling silver rather than over a base metal, at a minimum thickness set by trade regulation.
The silver core gives it more intrinsic value than a plated brass item, and the thicker required layer gives it more wear resistance than ordinary plating.
It remains a coating, however, and repeated polishing removes gold rather than restoring it, which is the opposite of how solid karat gold behaves.
Refiners treat all three as base metal
Recovering gold from a coating requires stripping or dissolving it, and the quantity present is usually too small to cover the processing.
That is why scrap buyers decline plated jewelry outright and will pay for vermeil, if at all, on its silver content rather than its gold.
Solid karat gold is the only form where the piece's weight translates directly into recoverable metal, which is the real dividing line between these categories.
Marks tell you which one you have
Stamps such as a karat mark with additional letters indicating plating or filled construction are the fastest way to identify what a piece is.
Unmarked items require testing, and acid or electronic testers read the surface, which means a plated item can initially test as gold.
Jewelers therefore file a small notch in an inconspicuous spot before testing, so the reading comes from beneath the surface rather than from the coating.