There's a lot that makes a beer. Mostly it is classified on how it is fermented and what is used in the blend. Mostly they use barely, but rice, wheat, and corn are also used. There's some other stuff used mattering on location as well. A lot of flavor comes from how it is fermented with hops, water, and yeast. Here's a link that describes it well Wiwpedia - Beer .

Personally I love Black & Tans. It's a mix of bass light ale and Guinness dark ale. Can also be used with other light/dark ales. But I also enjoy my Cerveza's with lime, or a nice lager. Oh heck, I love most beer.