Beers are brewed using the yeast
Saccharomyces cerevisiae (brewers' yeast). This needs a warmish temperature, and it sits at the bottom of the wort (brewing liquid).
Lager uses a different species of yeast, called
S.carlsbergensis (honest!). This floats on the top of the wort, and can work at lower temperatures. This used to be important in continental climates, as lager could be made during the winter!