The term popularly signifies people in, or people from families in blue-collar or manual jobs. Doesn't necessarily relate to income: many people who would describe themselves as working class - for example, oil workers, printers and builders - earn fairly good money. Does relate to heritage: if you are, say, a doctor, or a TV producer with half a ton of academic qualifications, or a professor, or anything normally considered middle class, you might still legitimately describe yourself as working class if you are from a working class background.