What's the difference between an ale and a lager beer?

An ale is made with a fast acting yeast that is active at warmer temperatures, while a lager is made from a slow acting yeast that is active at colder temperatures. So, the fermentation in ale occurs up near the top of the vat, in the wort (the mash that's brewed to form beer). But, the fermentation in lager occurs near the bottom of the vat, away from the wort. So, the flavors of an ale tend to be stronger and heavier, while the flavors of a lager tend to by lighter and crisper.