Korinthen are very small and dark raisins (named after Korinth, German for Corinth, the town in Greece that gave those special raisins their German name). Kacker is a noun derived from kacken, a slang term for moving your bowels. Taken literally, then, a Korinthenkacker is a person who produces nothing more impressive than raisin-like turds when going to the bathroom. Figuratively, and that's how the term is used exclusively, it's a pedant who hides his inability to see the larger picture behind an obsessive focus on small details. I say "his" because Korinthenkacker is masculine—Korinthenkackerin would be the feminine form.
The most recent time I saw the term used was on a German blog, where a commenter was called a Korinthenkacker because all he or she had to say was to correct another commenter's spelling. And my Facebook friend Richard Caldwell pointed me to a very instructive blurb on the etymology of kacken.
Word of the Month: Index