The term Sterling Silver is used when referring to 92.5% pure Silver and 7.5% of another metal. Fine Silver has a purity of 99.9% which tends to be far too soft for use in household goods and jewellery hence it has to be 'mixed' with something else to give it strength. When doing this it's important that the Silver retains its appeal and doesn't lose any of its value so care must be taken what it's mixed with. Although Copper is most commonly used, other metals can be just as suitable and the reason for using different metals, is to try to improve resistance to tarnish and to also eliminate the risk of firescale. The metals which can be used are far too numerous to mention and they can also include a variety of other additives. Although new alloys frequently appear, no one has yet emerged to replace Copper completely as the industry standard.
People who have an allergy to Sterling Silver in the vast majority of cases, are actually allergic to Nickel which is often used as an alloy. Hypoallergenic Sterling Silver is Nickel free hence it's popular amongst those with such allergies. A high percentage of Copper will cause Sterling Silver to tarnish more quickly so to avoid this, it's important to use as little Copper as possible.
Most countries around the world have their own system for hallmarking Sterling Silver and the main purpose for doing this, is to indicate purity, to identify the silversmith or company who made it, or to note the date and/or location of the manufacture. 925 is the most widely recognized international hallmark for and it confirms the purity of the Silver is 92.5% with an alloy of 7.5% Copper or another mineral.