Brad Plumer has an incicive comment about how social class connects to one's work — and wriggles out of the over-simple idea that higher income equals higher social class.
One way to define class — and this is hardly an original thought — is to look not at income but at power. Power in the workplace. Power in the world. The working class, from this point of view, can be defined as those who do their jobs under strict supervision, have little control over what they do or how fast they do it, and have no power over anyone else. Notice I picked this definition somewhat deliberately; these are precisely the sorts of people who, under labor law, can join a union. Obviously the definition's not hard and fast. I'm in a union, after all, because at work I technically get no input into the Mother Jones budget, and have precisely zero authority over any other employee. So that's the law. In practice, though, I do have the ability to hire, promote, and fire interns, I get to work at my own pace, and have wide discretion over what projects I want to pursue. So I'd put myself in the middle class, even if I make far less, income-wise, than many who would be considered working class. Intuitively, this classification makes far more sense than calling me “working class” and, say, a well-paid, unionized electrician “middle class.”
I like this. I have a tendency, when talking about class, to avoid talking about “middle” class for this very reason, and refer to “working” versus “professional;” I break things up into poor, working poor, working, professional, intellectual, upper professional, aristocratic.