I recently worked on a project where I had to edit all the generic ‘dashes’ in the text to conform to some rules about when to use a hyphen, an en-dash, or an em-dash. I wanted to write about them here so they’ll be easier to find the next time I need them.
Hyphen: Used to join words or to separate syllables of a single word when the entire word won’t fit on a single line (most software does this for you). You would also use a hyphen to create a compound noun, verb, or adjective (mother-in-law,eye-opener, awe-inspiring, respectively).
Dashes: The en-dash is called en-dash because it is roughly the width of the letter ‘n.’ It’s used to separate numbers in a range (4–5 pieces), or to describe the relationship between two things (Father–daughter dance, Detroit–Chicago train). There is a keyboard shortcut when using most desktop publishing softwares: hold down ‘control’ and hit the ‘minus’ key. This doesn’t work in WordPress, though. 
The em-dash is named because it’s roughly the width of the letter ‘m.’ It’s used to emphasize a parenthetical thought, or an interruption in dialogue instead of using an ellipsis. Examples:
- Please use my email address—shanhale at gmail dot com—for any correspondence.
- “I never said I wanted you to—,” she yelled into the phone, but he hung up before she could finish.
Em-dash also has a keyboard shortcut: Control + Alt + Minus.
Grammarbook.com has a nice page showing the differences between en-dash and em-dash.
