925 Sterling Silver is an allow of silver containing 92.5% silver and the remaining 7.5% is of other metals. Typically it is copper. The purpose for combining copper with silver is to strengthen and harden the durability of the metal. Pure silver at 99.9% is relatively soft and would not make an ideal metal for a ring for example. If you can imagine the frequency that you would grip a pan in the kitchen, laundry basket hauling clothes up and down the stairs to playing sports, the ring would soon reshape.
Care is needed with your 925 Sterling Silver ring like all fine jewelry. Tarnishing may occur do to the environments that it is exposed to. Using a sterling silver cleaning will keep your ring looking like you just purchased it. Only purchase from a reputable supplier like Jewelry Ice Box here.