Philosopher, journalist, economist, revolutionary — the 19th century's most consequential critic of capitalism and, uneasily, the patron saint of regimes he never lived to see. Marx began as a young Hegelian in Germany, moved through the humanist manuscripts of 1844 (alienation, species-being, the worker estranged from his labor), and in exile in London produced the mature critique: capitalism is a system that works, but its workings generate crises, concentrate capital, and grind human beings into interchangeable inputs of production.
Capital (vol. 1, 1867) is simultaneously a dense economic treatise, a moral indictment, and a historical drama. Its core argument — labor is the source of value, surplus value is extracted from workers whose wages are below the value they produce, and this extraction is hidden by the ordinary workings of the market — was Marx's attempt to turn Adam Smith's and Ricardo's political economy against itself.
He was also a historical theorist: material conditions shape consciousness, modes of production generate class struggle, and that struggle drives historical change. What he actually thought communism would look like he left strikingly vague; his writing on it is far less developed than his analysis of the system he wanted to overcome.