Although wheat, rye, oats, millet, sorghum, rice and corn have all been used for brewing, barley is the preferred grain for beer. But the starch in a grain of barley isn’t ready to be fermented into alcohol, so the barley is generally converted into malted barley, or “malt.” The process of malting involves soaking the barley, allowing it to germinate, and then stopping germination with heat.
The amount of heating that barley malt receives has profound effects on the sort of beer that will be brewed. All the color in beer comes from the malted barley. A lightly-roasted malt will produce a very pale beer. Deeply roasted malts produce dark or black beers.
So, take lightly roasted malt and make a beer from it. Use an ale yeast, and the result will be a pale ale, the classic English pub beer, or a bitter or golden ale. Use a lager yeast, and the result will be a style such as pilsner.
Use a malt with more of a golden roast, and the ale variety will be an amber ale or a Scottish ale; if a lager, perhaps it will be a Märzen, a festbier or an Oktoberfest beer—all beer styles with a slighter sweeter character.
Give the malt a little more heat, and the beers become darker, more the color of root beer. Brown ales—Newcastle Brown is a classic—are the ale variety. In the lagers, the cleaner tasting German dunkels—dark lagers—are the counterparts.
The popular wisdom is that these dark beers are stronger than light beers. On the contrary, the roasting may have the effect of “locking up” some of the starches in the beer so they cannot be fermented. There is less food for the yeast to turn to alcohol; the beers may be lower in alcohol, and the unfermented material stays in the beer, giving it a thicker texture in your mouth. The beers can feel rich, but actually may be less intoxicating than a mass-marketed lager.
More roasting. The next darker beers are porters and stouts (which are ales) and the rather rare schwarzbiers (black lagers). In keeping with the differences between the two families, porters will have a lot more spin-off flavors, such as fruity notes, than the schwarzbiers, which will be malty (sweet) but still very clean. Both styles acquire coffee or chocolatey notes from their dark malts. With stouts, the blackest of the ales, the addition of roasted barley can give the beer a burnt-toast edge.
Thanks to malt, lagers and ales both come in a full range of colors, strengths, and characters.