To explain the difference between gold filled and gold plated we turn to the federal standards for the sale and description of metals that are sold as commodities.
To simplify here you go.
Gold filled by definition must!!! legally contain at least 1/20th of 12K gold in the mixture of the metals of the jewelry item. It may contain layers of brass or other base metals. There is no outer coating so this jewelry is considered water safe.
Gold Plated can be over brass or silver or any other base metal. The product is dipped into a solution containing 14KT or 18 KT gold depending on the color preference. It sits in the solution for a short period of time until the molecules bond with the base metal.
Once removed the item is called Gold Plated. Due to the fact there is only an outer coating of gold on the product it is not water safe. In fact most mild abrasives remove the gold layer leaving the brass or other base metal to show through.
With care both can last a lifetime.