Gold-filled, or "rolled gold", is created by bonding or "welding" a thin layer of gold to a base metal core. This is achieved by using heat and pressure. USA industry standards require there to be 1/20th by weight layer of 14 karat gold permanently bonded over a core of base metal.
When you come across the reference "18/20 Gold-Filled" this means that item is 1/20 18K gold or more simply put the 18 = karats and the 20 refers to the % of gold in an item. 1/20 = 5% gold.
The 1/20 fraction refers to the ratio of the 18 karat gold layer to the base metal (brass) layer by weight, which is 5%
The base metal is most likely to be brass. 18k Gold Fill (14K GF) has ± 100x more gold than a gold plated item. The gold layer found on 18K GF items is usually 50-100x thicker than what is found on a gold plated item. Gold Filled is hard wearing and if looked after like fine jewellery it should last a lifetime.
Gold-plate is created using an electro-chemical process, a layer of gold is applied to a base metal by means of electroplating. This layer is usually quite thin, only a few microns. There are no industry standards to govern the quantity of gold or even the karat of gold used in gold-plate. Over vigorous cleaning can remove the layer of gold-plate. The wear factor of gold-plate can also be influenced by climate conditions and the person wearing the item.