"Sterling silver" describes an alloy that has silver content of 92.5%. "Silver plating" typically describes a very thin layer of silver that has been adhered on to a conductive object using electrolysis.
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.