Middle Class

I've been listening to an interesting series of stories on NPR this week about the notion "middle class." Turns out that if you ask almost anyone where they fall on the socio-economic ladder, the answer will almost universally come back "middle class." But if you ask "Are you lower class, working class, middle class, or upper class," the answers will split evenly between working class and middle class.

So the question they asked, which I'd like to play with here, is what does the phrase "middle class" mean? Are middle class and working class mutually exclusive or is working class a subset of middle class, or do they intersect in part? Is it purely defined by income or is it lifestyle?