The term "bock" in its modern usage, refers to virtually any strong lager; strong being over 6.5% ABV or so. While they are often dark beers, it is interesting to note that Michael Jackson points out that the original bocks, because of the high usage of wheat at the time and in the location, are actually what we would now call a "Weizenbock" or a "Wheat Bock." It was also top-fermenting (an ale) and fairly well-hopped (in excess of 30 IBUs).
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The bock was born in Einbeck, Germany sometime in the 1300s. As described above, however, it is painfully obvious that today's bock beer bear little resemblance to the original beer produced in Einbeck. Well, sometime in the 1700s, Southern Germans, because their beer (at the time) was so miserable, began to demand "that beer from Einbeck" - or, as they would have pronounced it in their fine southern drawls "Einbock." However, the southern breweries didn't have consistent access to wheat. The hard water in southern Germany was not very hospitable to hops, it made the already bitter hops, overly bitter (by contrast, the water in the hills and mountains of Northern Germany is much softer). Finally, the Southern breweries were far more familiar with lager fermenting than with the Northern top-fermenting. In short, it was the type of bastardization of a style that we continue to see today (to wit: the American wheat ale). And, the two styles could have competed with each other except for the fact some time around the turn of the 19th century, almost all of the breweries in Einbeck burned to the ground. With that, the world was left with the Southern German variation of the Einbeck-originated style: a dark, malty, sweet, lager. These days, beer brewed in the Northern style, lighter and hoppier, are designated "Ur-" (meaning "original").
So, why the spring?
Well, in the days before temperature control, even ale yeasts stop fermenting when the whether gets too warm. In Einbeck, these temperatures were usually reached in early May. So, in May, when the beer was done fermenting, a spring fair was held and the citizenry drank of this Einbeck beer.
And the goat often seen and associated with bock beers? Bock means "billygoat" or "goat" in German.
What about the dopplebock?
Believe it or not, the dopplebock originated in parellel with the bock, but is rather un-related. I'll let Mr. Bryce Eddings (you can read his blog, here) tell you about the Dopplebock:
Munich means “the home of monks” and it was so for the followers of St. Francis of Paula. These vegetarian monks from Italy observed two fasts each year – one during Lent and one for the month leading up to Christmas. It has often been told that European monks of this time relied on dark beers to sustain them through their long fasts and these Paulaners were no exception. They developed a particularly dark beer with a lot of protein and carbohydrates carried over from the mash that served them well during the times when solid foods were prohibited.Because these beers were so similar to the bocks, the public began calling them "dopplebocks" and so we have dopplebocks at Lent and Christmas. This original dopplebock recipe is still made today by the brewery carrying the same name as these monks: Paulaner and the beer is still called what it was then "Salvator." In honor of the "Salvator" most breweries christen their dopplebocks with names ending in "-ator" (e.g., Ale Asylum's Bamboozleator).
Some interesting reading about bocks:
All About Beer: Beer Styles: Bock
Beer.About.com (Bryce Eddings)
Michael Jackson, The Beer Hunter: Original Bock: The Beer The Doctor Ordered
Hoosier Beer Geek: the Knights dissect the BJCP bock guidelines