For the average beer drinker, the difference between an ale and a lager comes down to how the beer looks, smells, and tastes. Ales tend to be fruity-estery, while lagers are clean-tasting and frequently described as "crisp." But to a brewer, the difference is more fundamental than that. It's not color, or flavor, or aroma, or hop/grain/malt varietals or even water hardness that separates a lager from an ale. Simply put, lagers use an entirely different type of yeast during fermentation. All of the knock-on effects -- from different flavors and aromas to decreased fermentation temperatures -- arise from this difference. You'll hear some beer pedants describe the difference as "top-fermenting" (ale) vs. "bottom-fermenting" (lager) yeast, which is generally accurate, but useless to those who have no interest or experience with brewing.