According to eBay, if you find a tiny ".925" or "S.S.", it means that the piece is sterling silver and is stamped on jewellery to demonstrate the high quality of the product.
This stamp represents the fact that the piece of jewellery you are buying consists of 92.5% genuine silver. The other 7.5% will be alloy to help prevent tarnishing. Typically copper is used, although zinc, platinum or geranium, are also common.
When buying sterling silver jewellery, there are a few things you can do to make sure the metal is genuine. Check out some tips on how to check silver authenticity at eHow.com.