I was told the following story some years ago at the Cambridge University Library. Back in the 1920s the oldest attendant at the British Museum Reading Room retired, and it occurred to someone that he might be the last person to have seen Karl Marx when Marx was doing his research for "Das Kapital" there. So the old man was asked, did he remember Kark Marx? He thought a bit, and said, "Oh, you mean the German gentleman with the beard? Yes, I remember him: he used to come and read here day after day, for years. Then one day he left, and no-one ever heard of him again!"
The first time I went to the Soviet Union I retailed this anecdote to our guide, but I don't think she was amused.
(For a summary of Marx's ideas, see my earlier posting under the heading of Politics/Philosohy: Marxism)