What could be simpler than a little line on a page between words?

Style guides disprove this idea in detailed discussion of dashes and the humble cousin of dashes, the hyphen. Yes, 'dashes'.  There is more than one kind of dash...

The hyphen, though dismissed as merely the joiner of words (double-barrelled names, compound nouns and adjectives, and so on) is a powerful thing, though relegated to the footnotes of discussions about dashes. Using hyphens correctly isn't just a fussy thing proofreaders might correct you on but actually makes your writing more comprehensible, whether it's for day-to-day consumption in a bog-standard note, or in a piece of high-class literary fiction.

This is a nice simple explanation of how to use hyphens, and so is this.

As mentioned before, there are two kinds of dashes, and much is made of them. Their names are en dash and em dash. Back in the days of manual print setting, the en dash was so called because it was the width of the letter N, em being the width of the letter M.  In modern digital fonts, this tradition is maintained and, theoretically, whichever font you type in, the en dash (– ALT+0150) and the em dash (— ALT+0151) should match those same proportions.

The use of dashes, like other things, has diverged between American English and British English.  Americans still love their em dashes.  In the UK, we've kind of waved them goodbye.

In American-English texts you'll often see em dashes—whether in novels, newspapers or anywhere really—used like this where we would use en dashes and spaces.

So in British English the en dash prevails and you know how these are used – you see them everywhere – so you don't need me to tell you how.

Why I'm mentioning all this is because the blessed, humble, useful, hardworking hyphen often ends up being hauled into use as an en dash, if it hadn't got enough to do already. To make your writing look smart, be alive to the idea that - isn't – and you'll be amazed how lovely everything looks.

And just a note on hyphens and dashes when it comes to numbers. Ranges of numbers – e.g. pages 19–25 – use an en dash with no spaces.  Want to put in some dashes between numbers – e.g. in a phone number?  Use hyphens. 

By the way, the minus sign is neither a hyphen nor an en dash.  Use ALT+45 to produce it accurately.  But between you and me, I can't see the difference! - (minus) - (hyphen)