Originally Posted by Nathan

YDG it is "called" 'sz' or 'sharp s' in german. The difference between "Straße" and "Essen" for example is that the ß in Straße is more pronounced than the "ss" in Essen. It is more emphasized as one letter instead of 2. "Stra-ße" vs. "Es-sen". A single 's' is simply less sharp "rei-ßen" vs. "rei-sen" (to tear vs to travel).

Hope that helps