To elaborate, the Red Sox sold Babe Ruth to the Yankees (not traded, just sold for cash and a supply of baseballs I believe) following their last World Series victory in 1918. Babe went on the become, well Babe Ruth, as he became a full time position player rather than a pitcher with the Yanks, while the Red Sox had not won a single World Series since then until now.
Edit: Yeah, and the Red Sox always lose in hearbreaking fashion, which makes it a curse. See: the '86 World Series and Bill Buckner as someone said, Aaron Boone last year, two previous game 7's against the Cardinals.....