Argentium sterling silver is 92.5% silver, just as regular sterling silver is. But in argentium sterling silver some of the copper (which makes up the other 7.5% of regular sterling silver) has been replaced with germanium, causing it to be more tarnish resistant than regular sterling silver.
Another advantage of argentium sterling is that -unlike regular sterling- it does not get firescale (unsightly dark spots in the metal) when heated. This eliminates the need for chemical compounds used in the process of firescale removal, and makes for an environmentally friendlier choice of metal.
More on this at Wikipedia.