Gold Filled
A thick layer of gold mechanically bonded to a base metal, legally at least 5% of the item's weight. Far more durable than plating.
Gold filled is made by pressure-bonding sheet gold to a brass core under heat, rather than depositing a film electrically.
The standard requires the gold layer to be at least 1/20th of total weight, which is why stamps read like 1/20 12K GF.
It carries roughly a hundred times the gold of standard plating and can last decades without wearing through.
When you are buying
The best value in gold-look jewellery if you will actually wear it. It costs more than plating and a small fraction of solid gold, and it does not need re-finishing.
Gold Plating
A thin layer of gold deposited over a base metal. Gives the colour of gold at a fraction of the cost, and wears off over time.
Gold Vermeil
Gold plating over sterling silver rather than base metal, at a defined minimum thickness. The best of the plated family.
Micron Plating
The thickness of a plated layer, in thousandths of a millimetre. The single number that determines how long a finish lasts.