If history is the history of class struggle. what is the class struggle Marx is writing about?

If history is the history of class struggle. what is the class struggle Marx is writing about? How does he understand class? What are the class “camps” and how are they defined? How do the bourgeois maintain their power? Using the book: The Inequality Reader: Contemporary and foundational readings in race, class, and gender. 2nd edition BY David Grusky