The working class - term that causes the most confusion - involves everyone who works for a salary. This includes blue colar factory workers, white collar office workers, everyone working for the goverment etc. To belong to the working class says (for a marxist) nothing about your level of education, nothing about the kind of work you do. You work for a boss who pays you a salary. The working class is also the class that does not own its working tools.
This is a distinction of no real practical use, though. If you're a consultant who doesn't bother to open his own practice and does all his work as an employee of the NHS, you can make £100,000 a year, which means you're richer than many capitalists. Yet you would be in the same class as a minumum wage earner stacking shelves at Tesco. I don't see the use of such a classification.