Beer is beer of course, but for those curious minds out there, a Lager is a beer which ferments slowly at lower temperatures (40-52) from the bottom up (bottom fermenting yeast). Ales are fermented in warmer temperature (55 - 77) using top fermenting yeast. The yeast is the critical component here. Lagers use a yeast called Saccharomyces uvarum and Ales use a yeast called Saccharomyces cerevisia (which is also used in bread making and wine.) The advantage of this yeast strain is it is able to deal with varying environmental conditions (temperature, alcohol content etc.). It's called top fermenting yeast because as it flocculates or clumps together the yeast attaches to the carbon dioxide and floats up through the wort.
Lager yeasts grow less rapidly than ale yeasts, and with less surface foam they tend to settle out to the bottom of the fermenter as fermentation nears completion. This is why they are often referred to as "bottom" yeasts. The final flavor of the beer will depend a great deal on the strain of lager yeast and the temperature at which is was fermented.
No matter what type of beer it is, the yeast will directly impact the overall alcohol content. Because ale yeast is much hardier in higher-alcohol environments, it will survive into higher levels of alcohol, causing ales to have a higher alcohol content, this is evident in IPAs and double IPAs which we will examine more closely in a later post.
Lagers, due to the fragility of the yeast and its inability to "protect itself" at lower temperatures, the yeast remains active at temperatures below 39°F. It also attenuates sugars more slowly, causing the brewing process also move more slowly. It has a lower tolerance to alcohol and has the ability to ferment mebilose, a sugar that is not fermented by top-fermenting yeasts. These last three features allow more sugar to remain in the mix, creating a smoother, sweeter beer, with less alcohol content.
Of course in addition to the lower ABV, the drawback to lagers for smaller breweries is that since it takes longer to brew it is not as economically attractive as ales.