In 1914 Joseph Lannin purchased the Red Sox team and also bought the rights to bring Babe Ruth, a.k.a. the Bambino, to Boston. Lannin sold the team in 1917 to Harry Frazee for $200,000. The “astute” Frazee recovered a large part of the purchase price by selling Babe Ruth, considered by many to be the greatest player in baseball history, to the New York Yankees in 1918 for the then-astounding amount of $125,000. The Curse of the Bambino has haunted the Red Sox ever since.
Prior to Ruth leaving Boston, the Red Sox had won five World Series, with Ruth an important part (as a pitcher) of the 1915, 1916, and 1918 championships, whereas the Yankees had never even been in the World Series. The Yankees have since won 26 World Series (15 more than any other team), with Ruth being an essential part of the 1923, 1927, 1928 and 1932 championships.
Since Ruth's departure, the Red Sox have been to the World Series four times, each time losing 3 games to 4. The most dramatic defeat came in 1986 when, in game 6, the Red Sox squandered three leads in what would be their deciding victory, only to lose the game in the bottom of the 10th inning on a fielding error by Bill Buckner.
In another episode of the alleged curse, the Red Sox had a 14-game lead over the Yankees on July 18 of the 1978 season. By season's end, however, the teams were tied for first. A one-game playoff took place at Fenway Park on October 2nd. In the 7th inning, Boston led 2-0, but Bucky Dent, a .240 hitter with only 4 home runs all season, hit a home run over the Green Monster (the leftfield wall, only 310 feet from home plate at its closest) with two runners on base to secure the Yankee win. More or less the same thing happened between the Sox and the Yankees in the late 40s. DiMaggio versus Ted Williams. Mel Parnell and Ellis Kinder, pitching for Boston vs ? and ? for New York.
In 2003, the Yankees prevented the Red Sox from getting to the World Series. Tied at three games apiece in the American League Championship Series, Boston had a 5-2 lead going into the 8th inning. Two Yankee doubles and a single later, the game was tied. The game went into extra innings and was decided by a first-pitch lead-off home run by Aaron Boone.
The curse is such an established part of Boston culture that it has been the title of at least one book and was made into a musical play in 2001.