Karl Marx defined history through economic class struggle. He argued that society was divided between the bourgeoisie (middle class, business owners, and capitalists) and the proletariat (wage workers). His belief was that the bourgeoisie grew wealthier and wealthier by exploiting the proletariat. Marx theorized that a time would come where by the proletariat would rise up and overthrow the capitalists thus creating a classless society. Marx believed that wage workers were always paid less than the true value of their work. He defined the difference between the value of the wage and the value of the goods produced as surplus value.
Over time the frustration over exploitation would result in class war between the proletariat who believed the surplus value belonged to them and the bourgeoisie who were not willing to give up their profits.
In the end revolution was the inevitable result brought on by the endless exploitation of the workers by the capitalists.