Sterling Silver is an alloy of silver and copper. It contains 92.5% silver and 7.5% copper, by weight. This blending of metals makes it harder and more durable, thus making it more suitable for making jewellery, than softer fine silver.
Unfortunately, the sterling silver alloy is prone to tarnishing due to the copper component. Because of this, other metals, such as germanium, are also alloyed with silver to produce an alloy called Argentium that resists tarnishing. Argentium is used in jewellery production, but sterling silver is still far more common.
It is believed that sterling silver originated in continental Europe. Exactly when, is difficult to say.
Sterling silver marks do vary, but the most recognizable mark on jewellery is simply .925. However, you may also see sterling silver and just silver.