Sterling silver and 925 silver are actually the same. Both refer to a compound with 92.5 percent silver content, where the additional 7.5 percent is made up of other metals, most commonly copper.
Sterling silver, 925 silver, and 925 sterling silver are just three different ways to say the same thing. As with gold, silver is very soft, so the combination is necessary to properly create jewelry as well as designs on silver surfaces. Silver's propensity toward tarnishing is due to the copper content, which goes through its own process of oxidization, causing the discoloration. Various organizations attempt to create more tarnish-resistant silver, substituting or adding to the copper with titanium, palladium, niobium, aluminum, germanium, boron, zinc and zirconium. Another method involves shifts to the melting method, using varying temperatures, gases and containers to try to halt the oxidation process.
Pure silver, on the other hand, contains at least 99.9 percent silver, which greatly diminishes (and in some cases eliminates) tarnishing, as the presence of metals that oxidize is almost completely depleted. Silver plated refers to a piece cast in another metal, often nickel or copper, with a fine layer of sterling silver on the outside. The silver plating still oxidizes, and it wears off over time.