From: Daily Writing Tips
The en dash is the oft-neglected middle sibling of the horizontal-line family of symbols that serve to connect words and numbers for various reasons.
The em dash (—) is the dashing member of the brood, used somewhat sparingly to indicate a sudden break in syntax—either to signal a shift in sentence construction, as here, or joining with a twin to frame a parenthetical word or phrase (just as a pair of commas would be used in the midst of a sentence or two parentheses would be employed anywhere).
The smallest, the hyphen (-), is the busiest, indicating connections between words, such as when the phrase “highest scoring” is hyphenated to signal its combined modification of the word that follows in the phrase “highest-scoring player” or to link two numbers in reference to a score or vote.
The en dash (–), however, sometimes steps in to take the place of the hyphen: It is employed when an open compound is part of the phrasal adjective, signaling that the entire compound, not just the last word in the compound, is linked to the next word, as in “Civil War–era artifacts” (rather than “Civil War-era” or “Civil-War-era”) or “Los Angeles–to–San Francisco flight” (rather than in “Los Angeles-to-San Francisco flight” or “Los-Angeles-to-San-Francisco flight”).
Note, however, that open compounds need not be proper nouns, as this quip about an advertising agency with a name consisting of a sequence of initials demonstrates: “This alphabet soup–named firm helps get clients on the gravy train.” If a hyphen were used in place of an en dash here, the reference would (confusingly) be to a soup-named agency of an alphabet nature. (Also, some publishers, presumably for aesthetic reasons, employ en dashes in place of em dashes.)
The other major function of an en dash, by the way, is to replace to to indicate a number range, as in “Answer quiz questions 1–10.” (Remember that because scores are not number ranges, a hyphen is the correct symbol for linking two totals.) In both types of usage, a hyphen is often erroneously employed in place of an en dash (though for the sake of simplicity, some publications, especially newspapers, deliberately avoid use of the en dash).
Also, note that although both hyphens and en dashes are employed as minus signs, the minus sign is technically a distinct symbol that in formal publishing is set using a distinct code. In informal usage, an en dash, more equivalent in size to plus and equal signs than a hyphen, is preferable.