What are the main tenets or features of Marxism?

The main tenet of Marxism, as described by Marx and Engels themselves in their 1848 work Communist Manifesto, and in Marx's magnum opus Capital, is this: History is best understood as a series of class antagonisms. One's membership in a class, according to Marx, was defined by one's relationship to the predominant means of production in a society. By Marx's time, the Industrial Revolution had led to the creation of mills and factories as well as the complex financial institutions that made these businesses possible. So Marx thought that the Industrial Revolution had ushered in a new and final period of class conflict between what he called the "bourgeoisie," the owners of the factories and mills and the capitalists who backed them, and the "proletariat," or working class that did not own the means of production. Over time, Marx thought that striving for efficiency and higher profits would necessarily drive more people into the proletariat and further alienate them from the value of the things they produced. It would also lead to more wretched conditions for workers. Eventually, the industrial working class, the proletariat, would rise up and overthrow the bourgeoisie, just as that class had done to the old nobility. This would be, Marx thought, the end of history, as class conflict would end. The proletarian revolution would usher in a classless society, and property ownership would no longer be the standard for social class. Marx thought this process would be inevitable because of the relentless logic of the profit motive that drove capitalists to constantly seek higher efficiency, even at the expense of alienating the working class.