Rauchbier is a beer brewed with some smoked barley.   All beer used to be smoked beer.   The barley is dried after it is brought in from the fields over a beechwood fire, which imparts a smokey flavour to the barley & hence the beer made from it.   Only with the advent of kiln drying & the use of coke did the smokey flavour dissapear, leading to beers with more subtle flavours like pale ales & pilsners.

Modern rauchbiers use mostly regular brewers barley with a proportion of smoked grain to give degree of smokiness.   Aecht Schlenkerla is the both the most famous & strongest smoke flavoured beer.   This comes from Bamburg, which is the home of smoked beers in Germanyⁱ.

A subtley smoked flavour can give a porter or stout an extra dimension to its taste.   Some of the German rauchbiers have a strong smoke taste which is not unlike smoked Bavarian cheese - weird but good.