Residual Sugar or RS

Wine is made from very sweet grapes. It is the sugar in these grapes that the yeast changes into alcohol. If you stop the fermentation while there is still sugar in the wine you have residual sugar! Or, after the fermentation has finished, a wine may have sugar added back in. While this probably isn't technically RS, we tend to refer to RS as any glucose/fructose in the wine when we drink it.
RS can be used in any wine and for a variety of reasons. Obviously it can make a wine sweet which tends to work really well with the flavors of certain varietals like muscat. However in most white varietals a small amount of residual sugar is used to create balance with the acidity. In a dry red a very tiny amount of residual sugar can be magical in the way it adds to the mouthfeel and overall structure. In a dessert wine it gets to be the star of the show making the wine thick and sweet! So in the end, whether it is a reisling or tempranillo, It is the winemakers decisions that lead to how much RS is in any given wine and therefore how sweet that wine is.