Let's start off with good old Wahl & Henius:
"Besides the recognized types, like the Bohemian, Vienna and Bavarian beers, of each of which there are brewed two varieties, the Schenk or Winter Beer and the Lager or Summer Beer (see above), there are beers brewed for special purposes of each type like Bohemian Export, Vienna Export or Bavarian Export, or beers brewed for special occasions like Bock.
Export and Bock differ from the Schenk and lager in that they are brewed stronger and contain more alcohol. Thus the percentage of alcohol and extract found, as the result of the analyses of a large number of beers, was on the average:
Alcohol. Extract. Schenk or Winter Beer 3.36 5.34 Lager or Summer Beer 3.93 5.79 Export Beer 4.40 6.38 Bock, Doppelt or Marzen 4.69 7.21
"American handy-book of the brewing, malting and auxiliary trades" by Robert Wahl & Max Henius, 1902, pages 791-792.
Got that? If you haven't, go back and read it again. No point continuing until you have that clear.
Here's a little more on the differences between Sommerbier and Winterbier:
"To 1000 l wort 6-10 l yeast is added. The temperature of the wort when pitching averages 6-8° for Sommerbier, 9 to 11 ° C. for Winterbier, but once fermentation has started the temperature is cooled further, down to to about 4° C., by hanging in it metal balls filled with ice, to let the fermentation run slowly. After completion of primary fermentation, which lasts 9-10 days for Sommerbier, 7-8 days for Winterbier, the fermented wort, which is called green beer or Jungbier, is ready to be filled into casks."
"Brockhaus' Conversations-Lexikon, Part 3", 1882, page 32. (My translation)
And here's some more about the strength of different types of beer:
"The alcohol content, expressed in percentage by weight, is in Bavarian Winterbier 4 per cent in Sommerbier 4.5, in Schwechater beer 4.2, in Strasbourg beer 4.3, Pilsen beer 3.3, beer from Waldschlößchen in Dresden 3.2, Dresden Feldschlößchen 3.7, Berlin Tivoli Beer 4, Porter 6-7, and 6-9 per cent in Ale. The concentration of extract in the German beers to 4.4 g, 8.6 to 9.8 in Munich Bock, Salvator at 9.0 to 9, 4, 5.9 to 6.9 in Porter, Ale at 14 to 19.19 per cent, 11 beer contains 0.57 to 0.93 g phosphoric acid."
"Brockhaus' Conversations-Lexikon, Part 3", 1882, page 33. (My translation)
I would continue. But I don;'t like my posts being too long. I understand just how easily modern man is intimidated by blocks of text more than a screen long. More tomorrow. Including a couple of really groovy tables.