Das Kapital is a hell of a big book - as in three 800-plus page volumes. Like, really freaking huge. 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. While massive, It could have been MUCH bigger- Marx planned six books, but only really completed one before his death, with homeboy Engels using uncompleted manuscripts to complete two, three and four. The books main idea is the Labor theory of value- the value of a commodity can be objectively measured by the average number of labor time required to produce that commodity.
Marx himself in the French introduction of the book does acknowledge that the length of the books may put people off- he was right with that. Reviews of the book have been somewhat mixed, with some people like Ernest Mandel singing songs of praise for the book, while others have disagreed with the ideas inside. However, the impact of the book itself on the standards of the worker and the many movements it inspired can not be denied.
 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 professors of his day and writing newspaper articles agitating for all sorts of concepts.
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 40 years. This website provides video and audio of a semester-long course on the book by one of the world's postmodern critics.