It's just that unlike most philosophers and historians, he advocated for revolution. His greatest work, Das Kapital, sets out to explain the world of the 19th century in historical and philosophical terms. Marx's thinking is deep and dense and we're low on time, but I want to introduce one of his ideas, that of class struggle. 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.