Gold-filled wire is a tube of gold, (usually 14k, sometimes 12k or 10k), that has been filled with a base metal, (generally pure jeweler's brass), and the two are securely bonded together with heat and pressure. In the United States, the Federal Trade Commission sets the definition as to the quality of gold-filled. For example, if an item is stamped 10KGF the gold must be no less than 1/10 of the item's weight, and 12KGF = 1/20. In Europe, 'gold-filled' is called 'rolled gold'. In all gold-filled wire scenarios, everything that is seen or that will touch the skin is karat gold. The gold layer on gold-filled wire will not chip off or wash off. If a person were to wear a 14/20 gold-filled piece of jewelry every day continuously, the gold would last from 5 to 30 years!
Rolled gold 'plate' however, is just as it is called: plated. When a base metal is coated with a thin layer of gold via electrolysis it has been 'plated'. If the base metal used is sterling silver and 18K gold is bonded to it, it is called Vermeil. Gold plate will wear off quite quickly, and the base metal below will tarnish.
14k gold-filled items have 50 to 100,000 times more gold than regular gold plating, depending on the manufacturer, and 17 to 25,000 times thicker than heavy gold electroplated items. I have been using 14/20 gold-filled wire to make jewelry for many years and have never had any returned due to issues with the gold wearing or coming off in any way. (Yes, I also work occasionally in 14k and 18k, but that can be extremely expensive).