Lurking on most most Berlin bar menus, waiting to surprise the uninitiated, is the Berliner Weiße:
This is a wheat beer unique to Berlin which undergoes a lactic fermentation to produce its characteristically sour taste. As a result, it's almost always served with a shot (Schuß - 20ml in a 330ml serving of beer) of syrup to sweeten it - either raspberry (Himbeer) or woodruff (Waldmeister). Often you'll see them referred to just as 'rot' (red - raspberry) and 'grün' (green - woodruff). Napoleon's troops apparently called it the champagne of the north; whether they put raspberry and woodruff syrup in their champagne too wasn't recorded. Actually (more seriously), I once overheard a tour guide saying that the practise of putting syrup in Berliner Weiße comes from the Huguenots who came to Berlin in the late 1600s, though I suspect this isn't true and that it's actually a more modern practise (it seems by the 19th century they were drinking it with a shot of Schnaps or Kümmel, a caraway spirit; I'd guess the sweet syrup idea may not have become common until the early 20th century, but that really is just an educated guess), which neither the Huguenots nor Napoleon's troops would have heard of.
Berliner Weiße is low in alcohol (about 2.8%) and many find it a refreshing summer drink...or a refreshing winter drink, I suppose.
From Bavaria, in the south of Germany, comes the Hefeweizen:
This is also a wheat beer - Weizen being wheat, Hefe being yeast, which is what gives the beer its cloudy appearance. Its flavour is slightly fruity, with hints of bananas, though there's no actual fruit flavouring in it like the fruit syrup in Berliner Weiße. It's a more robust kind of beer, with more alcohol - usually around 5.5%. You'll also often see a Dunkelweizen (a dark version) and Kristallweizen (a filtered version, sometimes served with a slice of lemon).
There are of course many other kinds of beer commonly served in Berlin, both brewed here and elsewhere - you may find this post becomes the first in a series...