What characteristics are used to distinguish styles of beer from each other? There are many styles of beer. It's straightforward to discuss the differences between two types of beer, but what are the characteristics used to distinguish between styles of beer?

For each general characteristic, what kinds of beer have each specific quality?  For example, if color is a general characteristic what styles of beer are light, medium, and dark?
The BJCP Style Guidelines specify a wide variety of variables.  These include:


Aroma (malt, hops, yeast, diacetyl, etc)
Appearance (color, clarity, head, etc)
Flavor (sweetness, bitterness, dryness, alcohol, carbonation acidity, diacetyl, fruitiness, etc)
Mouthfeel (body, carbonation, smoothness, astringency, etc.)
Ingredients (yeast, mash bill, hops, added ingredients, etc.)


These guidelines are from the Beer Judge Certification Program, so they are by necessity based on idealized judging, rather than the intent to taxonomize all of known beer.

The first four are subjective, coming from the standpoint of categorizing based on the drinking experience.  The last, Ingredients, is objective and is used for quite a bit of general categorization (ale vs lager yeasts, for example).

You could also argue that the source of many of the subjective factors is variation in the brewing process, such as fermentation temperature, addition or subtraction of various steps, etc.  However if you come at it from the angle of "how do I categorize an unknown beer?", then the first four subjective items are key.  Some of the question is whether you focus on what it is or on how it got that way.