Palladium Sterling Silver
Sterling Silver usually contains copper. It is the copper that leads to the common tarnishing you see. Palladium Sterling silver replaces the copper with Palladium (one of the Platinum group of metals) and results in a tarnish resistant 925 (92.5% pure silver) Sterling Silver alloy.
Most white gold alloys are naturally a greyish colour. To achieve a white finish it is standard in the jewellery industry to plate white gold pieces with Rhodium, another of the platinum group of metals. Rhodium plating is not permanent and does wear off over time according to wear.
Palladium Sterling silver is a naturally white colour therefore does not require rhodium plating, is tarnish resistant and less expensive than 9ct white gold.