The working class is the "lower" class of capitalism the class that has nothing to sell but their labor power. This is contrasted with the bourgeoisie who are a class of people who own the means of production The working class makes up the vast majority of the population of the Earth.

Per Capital Vol. 2 the working class is anyone who is broadly speaking dependent on the wage fund that services the general reproduction of the circuit of capital. That is to say that the working class is not simply commodity-producing laborers. This is a common mistake that both Marxists and their detractors make.