Silver is a precious metal. It is highly malleable, and is the most reflective metal on the planet.
Because of this malleability, Silver can be easily worked into a piece of jewelry, but it could also be easily bent or deformed. That is why, silversmiths mix it with a harder metal to make it more durable. The alloy metal is usually Copper (most common in Mexican Sterling Silver) or Nickel, since neither metal affect the color or reflectiveness of the silver
.925 Silver or Sterling Silver: In order to be labeled Sterling Silver the alloy needs to contain a minimum of .925 silver, this is the most commonly used percentage for silver jewelry. Mixed with 92.5% Pure Silver is typically 7.5% Copper or 7.5% Nickel.
.950 Silver is also a combination of the same metal, and in this case the percentages are: 95% pure silver and 5% other metal. This type of silver tends to be a little whiter and shinier than Sterling Silver.
.999 Silver is basically 99% pure silver and 1% copper. It is also called Pure Silver or Fine Silver. This type of silver, because it tends to be softer it is only used in jewelry that requires weaving or that has a very intricate design.