What is "ß"?
I've seen it in the German forums. It looks like a fancy "B" to me.
'ß' is a ligature combining the long s - ( ſ ) - and the short s ( s ). ſ was still commonly used in English for initial and medial s until well into the 19th century; and a double-s (since at least one of them would necessarily be initial or medial) would be written as 'ſ s', which, closer together, easily looks like 'ß'. I don't know the precise history in German (they've undergone various spelling reforms), but in 18th century English books I have, 'ſ s' is also printed as a ligature, albeit a little less stylised than the German 'ß'.
the fancy "B" to which you refer results in what sounds like an English "s". The German "s" sounds like the English "z". The German "z" produces a sound which in English would require "tz". As others have mentioned, Germans replace it with "ss" if they are using a keyboard that doesn't support it.
The same thing happens with umlauts which result in a trailing "e" if the umlauts are not available. I have these at home but not at work. I don't have the fancy "B" with either keyboard.