Q: What's the difference between capitalism and communism? A: In capitalism, man exploits fellow man. In socialism, it's exactly the opposite.

(Note: original as in, the question was phrased in terms of what's the difference, and the answer famously began with "in capitalism, man exploits fellow man", which was a standard formulation of propaganda texts. There are, of course, countless variants, as with any joke.)

It was a tongue-in-cheek way of pointing out that, even though they claimed to have eradicated the exploitation of workers by the owners of the means of production, all that Communist regimes of the time had achieved was to replace it with the exploitation of workers by the state, for the benefit of a small elite that ran the whole system. Once you cut through the propaganda and the jargon, workers were still being exploited (in the Marxist sense) by a small group of wealthy individuals.

The exploitation of man by fellow man was a pretty famous, and often-invoked part of Marxist doctrine, and one of the first things that were covered as part of political education courses in schools, workplaces and so on. Besides making the point above, it also mocked the stiff, pseudo-scientific Marxist-Leninist propaganda speak. Most of the people in charge of "political education" were uneducated party activists -- the kind of people who would, indeed, say that "in capitalism, man exploits fellow man, whereas in socialism, it's exactly the opposite" with a straight face, without understanding the double-entendre.