Das Kapital is a hell of a big book - as in three 800-plus page volumes. Like, really freaking huge and not making much sense. It was co-written by Friedrich Engels and Karl Marx, the original Commies, and so no good God-fearing American needs to bother with it. For some reason, it is always referred to by this name, rather than its full name in German: Das Kapital: Kritik der politischen Ökonomie, or in English as Capital: Critique of Political Economy.
Lots of college students and other wanna-be radicals will claim to have read it. They are mostly lying. Those who have tried to read it will usually occupy mental institutions, having gone insane from sheer boredom. Or they're working economists, which is similar but you wear a tie to work.
 OK, Seriously...
Marx spent his entire life putting off finishing this book. Francis Wheen's biography makes it clear that, besides being a bit of a genius, he also was a bit of a procrastinating frat boy that refused to grow up. He was prone to nasty carbuncles that made him finish the book standing up. He had squandered all chances for ever becoming a tenured professor in Germany by aligning himself with the most radical, anti-authoritarian professors of his day and writing newspaper articles agitating for all sorts of radical concepts, including democracy and the right of poor people to pick up fallen tree branches for their fireplaces. That really went over terribly with the totalitarian Prussians, who thought they had been ever so nice for having allowed the Jewish student to go to college in the first place.
Marx was deep into F.W. Hegel, whose philosophy made perfect sense to him if you flipped it upside down. Hegel said "All that is real is rational", ergo God bless the Kaiser. Marx said "All that is rational is real", meaning anything that was notably un-realistic, like totalitarian Germans picking on Jews and limiting the freedom of the press, was inherently irrational, and that the Kaiser should abdicate while the workers of the nation united democratically. The issue of who was being more realistic then or now is another conversation.
After participating in the 1848 revolutions in Europe, Marx was exiled to Soho, London, while Engels ran his father's textile mill in Manchester. Marx told Engels to send him all his accounting and production figures, which formed a major backbone for his research. Marx also constantly exchanged letters full of filthy language and no-no words, along with a regular request for cash. The original idea for the book came in 1855, when Engels prompted him to get all his ideas out in one big go. After writing the Table of Contents for each volume, he spent twelve years smoking bad cigars, hellraising with drinking buddies, lecturing union halls, and writing newspaper articles, along with every other excuse in the book.
 See also
Reading Capital with David Harvey: David Harvey has been teaching a seminar on Capital for forty years. This website provides video and audio of a semester-long course on the book by one of the world's leading Marxist and postmodernist scholars.