There are three kinds of dashes: the hyphen (-), the en-dash (–), and the em-dash (—). Each of them have specific uses.
The hyphen is used to join words together into one entity. An example? He-Who-Must-Not-Be-Named or You-Know-Who.
Another use is after certain prefixes. For example: the mid-1920s.
Finally, you can use a hyphen to simulate stuttering within dialogue:
"This apple is b-bruised!" Sally stammered.
However, the main reason I made this chapter is because people often use hyphens where an em-dash should be used.
An em-dash is the lengthy dash characteristically seen at the end of a sentence which is cut off or to separate elements of a sentence like a comma would.
"I'm going to get a new-" Sally began, but the cashier threw the apple at her face-blinding her-and she stumbled backwards into the candy wrack. is incorrect. It should be:
"I'm going to get a new—" Sally began, but the cashier threw the apple at her face—blinding her—and she stumbled backwards into the candy wrack.
But how do you even use an em-dash when it's nowhere to be found on your keyboard? Well, you could use an alt-code every single time you write it, but that's no fun, so I'll show you how to program Microsoft Word so that you can use the em-dash as quickly as a hyphen.
Open a blank document and go insert > symbol > more symbols. In the subset menu, you can find 'General Punctuation'. Press that and find the EM-DASH. Once it is selected, there should be a button in the bottom left corner of the box that says, 'AutoCorrect...'. Click on that and it will lead you to a page that allows you to replace certain characters with an em-dash. I use two hyphens, as you would never have a use for two hyphens anyway. Press 'Okay' and now every time you type the hyphen key twice, it will auto-correct to an em-dash. Neat!
On a final note, the en-dash is used to signify a range in elements, like:
Sally bought apples from Mark's from 1998–2015.
Pages 9–103 of the apple-buying manual had absolutely no instruction on what to do when encountering a bruised apple.
Honestly, though, just use a hyphen with a space on either side. It's one of those things that most people look over.