"A spectre is haunting Europe…"
In the popular imagination Karl Marx is many things. For some, he is a terrifying evil spirit to be ritually exorcised and slandered and derided with religious zeal. In their minds -- it seems -- Marx embodies that greatest of all threats: communism. BOO! :-)
For others, "Marx was a socialist theoretician and organizer, a major figure in the history of economic and philosophical thought, and a great social prophet."
That's how the "Introduction to Sociology", hosted by the School of Social Sciences of Cardiff University, presents Marx to his students.
In addition to those other things just mentioned, Marx just happens to have been one of the three founding fathers of sociological thought (the two others being Emile Durkheim and Max Weber, plus Georg Simmel -- an often forgotten, but sometimes considered fourth father -- all of which, plus Antonio Gramsci -- another Marxist -- and a few other thinkers are featured in that website).
Marx's section contains a sub-section with a brief biographical profile ("The Person"), sketching his personal background, intellectual evolution, and political work and a very instructive sub-section summing up his work as it relates to sociology only ("The Work"), where the emphasis rests upon "Overall Doctrine", "Class Theory", "Alienation", "Sociology of Knowledge", and "Dynamics of Social Change". The exposition is brief and jargon-free, requiring no previous knowledge of Marxist or sociological terminologies, but demanding an active and careful reading.
Valuable as that content is, there are some very important things missing: Marx's theory of exploitation, founded on his law of value (or labour theory of value) and Marx's attempts at a theory of crises. Perhaps the author of that site considered, not without some reason, that topics like those were more appropriate for economics than for sociology. It's still a regrettable omission, from the point of view of those interested in the whole of Marx's thought..
With that limitation in mind, I'd recommend that site without any further reserve.
Personally, I liked how sociologists summarised Marx's historical materialism or materialistic conception of history in "Overall Doctrine".