Argentium Sterling Silver is a patented and trademarked alloy that is at least 92.5% pure silver like regular sterling silver. What makes it different from regular sterling silver is that a small amount of germanium replaces some of the copper that is usually the other 7.5% of a sterling silver alloy. Peter Johns, a professor of silversmithing at Middlesex University in England, invented it in 1996.
The main advantage of working with Argentium Sterling Silver is that it is not subject to firescale like normal sterling silver. Although it does oxidise like sterling, this is easily removed by pickle. It is also more resistent to tarnish than sterling.
For more information, you can ready an article about Argentium® Sterling Silver by Cynthia Eid, published in the Society of North American Goldsmiths Tech News, and available on her website.The official site by the manufacturer are ArgentiumSilver.info. You might find their Questions and Answers section particularly helpful, as it covers the most common technical questions.