Gold-filled is composed of a solid layer of 14k gold, which must constitute at least 5% of the item's total weight, mechanically bonded /plated to a base metal (alloy of brass or copper).
18kt Gold plated is a thin layer of 18kt gold plated over a brass base.
Gold Vermeil (pronounced ver-may), is composed of a solid layer of 22-24k gold, mechanically bonded / plated to a sterling silver base.