Ah, the old “hyphen-versus-dash” confusion reared its ugly head again, and again, and again at a conference I attended this weekend. And, friends, it was ugly indeed. There is a vast gulf between a dash and a hyphen, and so I am going to set the record straight, right here and right now.
Let’s see. First off, they are two different marks of punctuation, with two completely different functions: a dash (—) separates; a hyphen (-) joins.
A hyphen joins words in what’s called a compound noun: spic-and-span, break-in, set-up, rip-off.
(Interestingly, and not to make you crazy, in the verb form of the compound noun, you don’t use a hyphen: I was ripped off.)
You often see a hyphen (-) when you join two adjectives. Hyphens join two, heretofore unrelated adjectives and create a whole new taste sensation, like, well, mint-chocolate. You take your mint,  you take your chocolate, you mix ’em up, and voila! You got yourself some mint-chocolate! Yeah, but here’s the deal, you only use the hyphen when this new combination is placed before the noun (stay with me here!) you’re modifying. So, it’s
mint-chocolate ice cream, but
the ice cream was mint chocolate.
Example: I love chocolate-covered cherries. (I really hate ’em. Yuck!)
But! I love cherries that are chocolate covered.
See? You use a hyphen when the two adjectives are placed in front of the noun, but there is no hyphen when the adjectives follow the noun.
Another example:
Liz hopes to become a well-known author. (Well + known modifies author.)
However,
Liz hopes to become an author who is well known.
If you say these two sentences out loud, you can actually hear the hyphen, and you can hear when you don’t use it. Try it.
Here’s an important fact about the hyphen. You never hyphenate an -ly adverb. Never. So all these terms are WRONG:
seriously-flawed,  heavily-decorated, flawlessly-cut, perfectly-centered
I know they all sound okay, but it’s wrong. Wrong, wrong, and wrong.
There are some prefixes that require a hyphen, regardless of whether the terms are before or after the noun they are modifying. A good example is “self,” which always takes a hyphen. (I look this up in my book all the time.)
Here’s a super-cool rule (a rule that’s super cool) about the hyphen: the suspended hyphen rule. If you have two or three words that use the same prefix or suffix, you suspend the hyphen: pre- and post-event, brothers- and sisters-in-law, right- and left-brained. This rule even works when there’s no hyphen! Example: Air Force One and -Two, bridesmaids and -matrons, upper- and lowercase letters.
Okay, now to the dash. A dash (—) can be created in two ways: you can hold down the ALT key and press 0151 on the number keyboard to the far right, or you can type a word in Word, press hyphen hyphen NO SPACE type the next word, and, when you hit the space key, your dash is created! Wow! Like, magic!
A dash creates a complete break in the action. In fact, if you remove the content after a dash or between two dashes, you should have a sentence that makes sense.
Here’s an example of a dash in action.
Liz flawlessly demonstrated the use of a dash—leaving her audience speechless.
Here’s another example:
My cousins—Tina, Jeff, and Kim—arrived late and left early.
I don’t use a space in front of or behind my dashes. I see people doing it, and I don’t mind, just as long as a) there’s a space before and after, and b) you’re consistent.
Last thing: two hyphens do not a dash make. There was a “ghostwriter” at this conference, and, naturally, I immediately went to the company’s website, to see how good this person really was, and I was shocked—shocked—to see two hyphens used when what she needed was a dash. Twice. Honestly. And don’t get me started about the fact that she used directional and nondirectional quotation marks…in the same sentence. Yes, I know you are shocked, but it’s true. I turned away in disgust.