Ravi, you can as well ask 'what is god, or patriotism, or being one of the pack?' 'Working class" is a secular expression of similar yearning - it does not have any empirical meaning but simply expresses the feeling of being "us and those who are like us" as opposed to "them." An $150k software engineer lives by selling his work - but he does not have the attributes of the American blue collar vernacular and may be even foreign-born, so he "aint' one of us" hence he is a yuppie not a working class.
In short - you or I will never be 'working class' in this country by the simple fact of being educated and foreign-born, and will never be able to convincingly emulate the blue collar vernacular that is an essential element of being 'working class.' Stated differently, no matter what's your or mine source of income or politics - we will always be asked the question "where are you from?" by members of the 'working class' - a sure sign that you or I are not one of them.
Wojtek