[Szass-Tam]

Well, this is the way I see the class breakdown, or certainly in England.
Traditionally, the working class were people who lived off their manual labour (the majority of the country for most of its history).
The middle class were people who stilled lived off their labour, but not manual labour, in offices and so on, bank clerks etc.
The upper classes were people who did not live off their labour and inherited money or owned property that made its own money (always a minority.
These definitions however are mostly economical, and as Lansera said, these don't hold true. Therefore, people probably define themselves as the class they were born into and take pride in being of that class (particularly the working classes).
These definitions are probably a little dated, but I find it easier to define them historically rather than in the present.