Basically a Dunkel with Rye instead of Wheat. http://www.germanbeerinstitute.com/Roggenbier.html
Roggenbier is a medieval ale usually made from a grain bill of about half barley malt and equal portions of wheat and rye malts. Today, a Roggenbier may be either an ale or a lager. Modern renditions of the brew have about 5 to 5.5% alcohol by volume. Rye ales are mildly hopped, which allows the grain flavors to be dominant. Filtration appears to be optional in a rye ale and many, such as the Paulaner (depicted right) are "naturtrüb," meaning naturally turbid. A yeast-turbid Roggenbier is more authentic, considering that the style had been around long before beer filtration was invented in 1878.
Being ancient brews, Roggenbiers can have a faint whiff of earthiness in the nose that is reminiscent of rye bread. The up-front sensation is one of mild fruitiness. There is a slight to extreme yeastiness and breadiness in the middle, and an almost smoky, spicy, faintly sour and very dry finish—clearly the effects of the rye malt. Effervescence ranges from medium to spritzy like a Hefeweizen. The body is substantial, almost reminiscent of a Bockbier. The brew has a pleasant, rich, off-white head when poured.
For the most part, Roggenbiers are tart, refeshing summer quaffing beers, a nice alternative to a Hefeweizen. They go extremely well with a succulent slice of barbequed roast pork.