TL;DR:
Em dash (—): Can replace parentheses (in pairs) and/or a last comma or colon. Mac: shift+alt+dash.
En dash (–): Separates numbers in ranges. Mac: alt+dash.
Quote dash (―): Indicates the speaker or author of a quotation.

Most readers intuitively distinguish between hyphens (-) and em dashes (—) as they make their way through a text. And most readers know that hyphens join coordinate words, creating compound adjectives and the like. But what are em dashes good for? And en dashes? What are quote dashes?

Em dashes can be used in pairs—like highfalutin parentheses—to set off tangential information from surrounding text. They can also act as highfalutin commas (or sometimes colons), to set off emphatic conclusions—the more emphatic, the better.

When tastefully and correctly deployed, em dashes can be distinguishing marks of good writing; but when overused they can be a distracting nuisance, lessening impact. Use em dashes at the ends of sentences to set off information that truly packs a wallop: “It was then that Detective Peters understood the truth. Ms. Hatheway hadn’t only lied to save herself. She had set him up—willfully.“ An em dash where a colon should be can have a comical, deflating effect. “Today I think I’ll go to Prospect Park, find the sunniest patch of grass, and eat my favorite sandwich—a turkey club.” Using em dashes in place of commas or colons might make you feel chic, but overusing them is a bad habit.

When it’s time to type an em dash, make sure to use the proper unicode character. Do not just string two hyphens together (–) even if your browser or word processor automatically reformats them into some kind of Frankendash (as yours may have done just now). Using a hyphen (or hyphens) in place of an em dash is déclassé.

The en dash (–) is the diminutive cousin of the em dash. It’s primarily used with numbers but only when those numbers are contiguous, or represent a range. For example: “His collection of bottle caps, amassed over nearly three decades (1971–99) was photographed for the Miami Herald.” Do not use a hyphen to indicate a range, especially if your readers are familiar with style norms for data (or obituaries). En dashes are slightly longer than hyphens: “John Lennon (1940–1980)” vs. “John Lennon (1940-1980).” Can’t tell the difference? OK, but your boss at the Financial Times can, so make sure you get it straight when it counts.

Alas, the en dash has its quirks. It can be used in place of the word “to” in limited cases, such as: “Take the New York–Philadelphia express train.” In very limited cases, the en dash does the job of a hyphen, joining an open compound to a third word to form a three-word adjectival compound (“The archaeologist took her time displacing the fault rock–heavy topsoil.”). If you need to learn more about this rule, you probably owe yourself a copy of The Chicago Manual (see 6.80). And just when you thought it couldn’t get any more confusing: Publishers in the UK (Oxford University Press excepted) prefer an en dash with a space on either side to an em dash in running text.

The quote dash (―) is perhaps the most obscure dash of all. Also called a “horizontal bar” (I wonder why?), it’s so uncommon (yet elegant) that it doesn’t even have its own keyboard shortcut. It looks almost identical to the em dash, but it’s not. Distinguishing between these two marks is an art (and a good excuse to break out the loupe, if you’re editing hard copy). Use a quote dash to indicate the speaker or author of a quotation. So:

The fault, dear Brutus, is not in our stars / but in ourselves. . . .

―Cassius, Shakespeare’s Julius Caesar

Still confused? Have some sacred knowledge on the subject of dashes? Leave a comment below.