Introduction
Marxism does not define a political system, it provides a very specific analytic framework for examining economic relationships under capitalism. Within those relationships lie antagonistic forces, those forces, labour and capital and, the struggle between them, create change.
And the point of it all....
The Theses on Feuerbach, (1845) tells us "the philosophers have only interpreted the world, the point is to change it".
That's it, there's no more. Simply put, Marx felt that philosophy's validity was in how it informed action.
Marx's own thought was shaped both by the revolutions and struggles of the mid-19th century and the philosophical quagmire of Hegelian philosophy. Marx spent half of his time arguing with intellectuals across Europe over the minutia of meaningless philosophical points, he spent the other half hiding from secret policeman.
Is Marx relevant today?
In 1867 Marx wrote in Capital....
"Within the capitalist system all methods for raising the social productiveness of labour are brought about at the cost of the individual labourer; all means for the development of production transform themselves into means of domination over, and exploitation of, the producers; they mutilate the labourer into a fragment of a man, degrade him to the level of an appendage of a machine, destroy every remnant of charm in his work and turn it into a hated toil; they estrange from him the intellectual potentialities of the labour process in the same proportion as science is incorporated in it as an independent power; they distort the conditions under which he works, subject him during the labour process to a despotism the more hateful for its meanness; they transform his life-time into working-time, and drag his wife and child beneath the wheels of the Juggernaut of capital."
This was capitalism in 19th Century Britain, this is capitalism in Africa, India, China, and Latin America today. Marx's analysis is still relevant. From the tin mines of Bolivia to sweat shops of Munbai, the labourer is still being mutilated into a fragment of a man.
Conclusion
Marxism is then not concerned with Soviet five year plans and the Gulag, rather it's concerned with understanding how capitalism continues to constantly reinvent itself through the consent of the masses.
Capitalism is not just about economic relations, it is also about a host of social relationships that foster fetishism and estrangement - bizarre forms of behaviour antithetical to human development and social progress.