For marxists there are three classes: the working class, the capitalist class and the small entrepreneurs (including farmers).
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.
The entrepreneurs are the small shopkeepers, the farmers, the very small businesspeople. The own their working tools, they don't work for a salary, but live from what their business earns them.
The capitalist class is the class that owns the very big majority of the working tools: they own the factories, the offices and sometimes big parts of the farming land. Basically they don't get their income from their work - in contrast with both former classes - they get their income from what they own.
As such I belong to the working class.