Demi fine jewelry is what is categorized as gold vermeil, gold filled and silver filled. It is a base metal that is then bonded with the precious metal.
Gold filled jewelry has a solid layer of gold that has been pressure bonded onto a base of quality jeweler's brass. Gold filled is legally required (in the USA and Canada) to be at least 5% gold (1/20 of the total weight), which is significantly more (easily 100x more) than gold plated items.
Gold filled is durable and will never rub or flake off. Under normal and extensive use, gold-filled will not tarnish. Under extreme conditions and when exposed to significant chemicals, like sulphur, gold-filled has been known to tarnish. It has a thick layer of solid 14k gold that offers a wonderful, quality piece without paying the solid 14k gold price.
Gold Vermeil jewelry is a demi-fine jewelry piece. The base is always silver, either pure or sterling silver and then a thick layer of gold is applied on top via plating. To meet US standards, the gold overlain on top of the silver must be at least 2.5 microns thick, and at least 10k or higher.  Vermeil pieces can last years if taken care of properly.