In US Currency Numismatics, a "Silver Certificate" is a special billet issued by the Bureau of Engraving and Printing/US Department of the Treasury from late in the 1800's to early in the 1900's (circulated until the 1950's). In much the same vein as the British Pound Sterling, a silver certificate was worth it's face value in Silver from the Treasury, and you could redeem it as such.
In the late 1920's, the US government started to poise the SC's not as an alternative to precious metals, but as a replacement for them; as a way to phase out common usage of silver coinage which, at the time, was real silver and as the nations population grew was getting harder and harder to come by in the quantities needed.
Up until June 24, 1968, you could exchange silver certificates for .77344 ounces of pure silver (the amount of silver contained in a silver dollar) per dollar in the form of silver flake, granule, powder or ingot. However, since that date has long since passed, they have no official value outside their stated face value. Except to collectors, of course.
As far as visual differences go, the markers of a post-1928 (after the small size bill changeover) silver certificate will be the words "Silver Certificate" in the top "facing side" border, and the Treasury seal and serial number will be in blue ink as opposed to the traditional green. Some rarer SC's will have the seal and serial number in yellow; these were used during WWII for US troops in the European Theatre.
Appraisals notwithstanding, Silver Certificates will have little value over their face value with notable exceptions of high-valued notes ($50+) and so-called "replacement" notes which have an asterisk (*) as part of their serial number.
And before you ask, there were also "Gold Certificates", but they are quite rare and were not generally printed after the turn of the 1900's.