Gold-filled is an alloy containing 14 karat gold and other types of metals (about 5% of the alloy is gold). Gold-filled 'behaves' like gold and does not lose color.
Gold-filled as its name is, gold filled with metal, used to create jewelry.
Gold-filled is gold that is attached to the base metal, such as brass. Gold is the outer layer, actually the coating, usually between 10 and 14 karats. The name originates in English and means 'stuffed gold'.
Gold filled jewelry contains at least 5% ... 12 karat gold and above, or at least 10% gold 10 karats, so that the coating is thick enough to prevent peeling, oxidation, and blackening.
Because 12 karat gold contains about 50% pure gold, gold filled jewelry will contain about 2.5% to 5% gold. Copper or brass is usually used for the inner layer, so you get a metal that looks like gold at a much lower price.
In the past sterling silver also served as a base, but being more expensive is not commonly used today.
Gold filled jewelry can withstand about 30 years in daily use without significant abrasion and can also be worn by people with sensitive skin.