You may have seen a stamp on a piece of jewelry or a tag that says .925 and wondered what that means. It simply means that it's sterling silver. Sterling silver is an alloy (mixture of metals) that must contain 925 parts out of 1,000 (92.5%) of pure silver. The remaining 7.5% is usually copper, but can be other metals depending on the manufacturer. (This is the standard in the U.S. Other countries have different rules for what they call sterling silver.)
Why add other metals to the pure silver? Just like pure gold, pure silver is very soft and is easily misshapen and dented, so it's not very practical for most jewelry applications. Some jewelry has components of pure silver (also called fine silver.) This would be considered .999 silver, or 99.9% pure silver. For example, the strip of wire around a stone called the bezel is often made of pure silver because it's softer than sterling silver and can be more easily pushed onto the gemstone without damaging it. But in general, the standard for quality silver jewelry is to use sterling silver. Even though it's a little high maintenance when it tarnishes, it can always be brought back to it's beautiful silver color.
So when you're buying a piece of jewelry, look for a stamp hidden on it somewhere. Sometimes it's so small that you can barely see it without magnification. A jeweler's loupe is great for looking for hallmark stamps, but a regular magnifying glass can work too. Common stamps that mean sterling silver include: .925, STER, STERLING, and STG. When a piece of jewelry is sent in for repair work the jeweler always looks for the metal stamp to determine what they are working with. So when you see a piece of jewelry is marked ".925" or "sterling silver" it's a good thing!