Sterling silver is .925 percent silver, whereas silver plated refers to a base metal, such as copper, that has a silver finish on top of it. Sterling silver can last forever if taken care of properly, unlike silver plated objects that can tarnish and fade over time.
Sterling silver is a strong metal, and only .75 percent of it contains other metals, such as copper and nickel. It is durable and long lasting, unlike pure silver and silver plating. Silver plated items are durable due to the metals that are underneath the silver plating, but the silver plating that covers the main metal can wear off.
Because "fine silver" (99.9% pure) has a softness that often makes it unfit for practical use, other metals, usually copper, may be added to silver to make the metal more functional. Thus, "sterling" makes a useful label, as it denotes silver of 92.5% purity. This silver is pure enough for beauty and ductility, yet hard enough for practical use. The etymology of the term "sterling" has not been definitively traced, but might relate to Old French terms for silver pennies.
Silver plating typically involves using an electrical current to adhere a very thin layer of silver to an electrically conductive object. This can be done for a number of purposes. For example, silver-plated jewelry has the appearance of finer silver, but can be sold for a lower price. Silver plating also has uses in electronics, as silver is very conductive to electricity.
The term "sterling" describes the purity of silver. Thus, this term has little to do with silver plating. True, some objects may be plated with sterling silver, but objects may also be plated with silver of lesser purity. Because silver-plated objects usually have only a very thin coating of silver, a silver-plated object and a sterling silver object of the same size likely have very different values.