The use of low temperature, top fermentation brewing techniques that make lager weren't developed until the 19th century. Obviously, however, places like Bavaria and Bohemia had been brewing beer for centuries before that. What would that beer have tasted like?
This is a great question. I once did a podcast episode (history of Germany) on the German Beer Purity Act.... and also an episode on Czech beer (Bohemican podcast)
So before lager all beer was BASICALLY top fermenting ale IN GERMANY. Before the beer purity act (and other countries like England, Belgium, etc) you get all kinds of things that preserve beer instead of hops. Even things like funguses and soot! But then hops was agreed on (and barley, malt) Really good beer was made with wheat instead of barley. The reason this changed is because wheat was reserved for bakers. But wheat beer existed the whole time and is still around today (hefeweizen or weissbier) ...so a HUGE variability until the 15th century or so, then it was narrowed down to malt, barely and hops, and then in the 19th century yeast was added to the list when it was discovered) And then 1948 all the citizens of Pilsen decided to throw in together and found a brewery. They hired a Bavarian brewmaster, who wanted to try bottom fermenting with a new type of yeast.
The "Lager" (storage areas) in Pilsen are a network of man-made caves under pilsen that go on for miles. They were kept even cooler by keeping ice from ponds in there.
This gave it a much crisper, lighter beer with less after tastes, etc.. it's basically the beer we know and love.
If you want to know more, I answered what the difference between Budweiser and Budweiser is (Annheuser-Busch vs. Czechvar) since Budweis is in the Czech Republic (like Pilsen)
And all of this I broke down much better here (audio) http://en.historyofgermanypodcast.com/?p=112