So for Marx, the focus isn't on the class, it's on the struggle. Basically, Marx believed that classes don't only struggle to make history, but that the struggle is what makes classes into themselves. The idea is that through conflict, classes develop a sense of themselves, and without conflict, there is no such thing as class consciousness. So Marx was writing in 19th century England, and there were two classes that mattered, the workers and the capitalists. The capitalists owned most of the factors of production, in this case, land and the capital to invest in factories. The workers just had their labor. So the class struggle here is between capitalists, who want labor at the lowest possible price, and the workers, who want to be paid as much as possible for their work.