I'll bet you've seen things advertised as gold filled and gold plated & wondered what the difference is. In short, gold filled is better. Here's why:
In the US, the Federal Trade Commission dictates that the gold in gold filled items must make up 1/20 the total weight of the item. Gold fill has a thick layer of gold on top of either sterling silver or another base metal like brass. Unlike plated jewelry, which has a relatively thin layer of gold over a base metal, gold filled pieces have a gold layer 5-10 times thicker than plating. Because of this, gold filled jewelry can last 10-30 years if cared for properly!
Gold plated jewelry is not terribly awesome for those with sensitive or particularly acidic skin (like mine). I cannot wear anything plated - within a few hours, the gold will be completely gone and my skin will have turned a not-so-lovely shade of dark green. On the other hand, gold filled jewelry is generally safe for sensitive skin. I wear my gold filled jewelry daily and my skin does not react.
Gold filled items are the next best thing to solid gold jewelry. It's high quality and beautiful. Will gold filled last forever like solid gold? No, but with proper care you can wear your gold filled jewelry for many, many years!
See also: How to care for your gold filled jewelry