When we talk about class we are referring to the belief that society is split into different groups, with different interests. "Class", that is a group of people in the same economic position, tends to cut across artificial boundaries such as race, gender etc. as fundamentally it is in the interest of all workers (the working class) to have more of a say in the way that society is run. Fighting against the interests of the workers are the bosses and the rulers who act as their representatives (the ruling class). It is quite clearly in the interests of this group of people to keep wages low (so as to leave larger quantities of money as their own profit) and to ensure that all those opposed to their agenda are not permitted a say in how society is run.