I was recently asked what the point was to the "em dash vs. en dash" argument. And although I've found the Internets don't take kindly to the finer distinctions of dash-hyphen relations, that won't stop me from posting my answer for you here.
An en dash conveys continuity. The party will be 10–?. Kids 4–6 may attend. The season finale of Lost will be on 8–10. The en dash connects, it bridges, it slides, it moves. On a tombstone, the en dash between the year of birth and the year of death represents those precious years that constituted life.
An em dash signifies a break. It is the dramatic pause, written. Obi Wan never told you—I am your father. Amy Grant—the fairy godmother of contemporary Christian music—is right here with us tonight. I'd like you to do something for me—drop dead. The em dash lets the reader know something big is coming. It introduces that parenthetical phrase that carries too much weight to be considered a side note, and it bows out that golden nugget of truth with more flair than a closing parenthesis but without the casual subtlety of a pair of commas. It stops the eye in its tracks for the delivery of an important message in a way a colon could only hope to do.
And then there's the hyphen, which indicates rapid-fire succession of words. It's what happens when two words hook up. If they stay together long enough, they might just get married and become a new word all their own. Birth and day hitched up to form birth-day and became birthday long ago. In the world of words, it was bigger than Prince Charles and Lady Di.