Using Dashes

Inserting a proper dash:

1. hold ALT and press 0151 on the keypad

2. for Microsoft Word: hold CTRL and ALT and press the minus sign on the keypad

If you can't insert a proper dash, use two hyphens: --

Usage:

Note: Don't overuse the dash, use other punctuation wherever possible, and never put spaces before or after a dash.

1. to set off parenthetical material:

He was a figure skater—though, like a fish from water, when on dry land he approximated a hippo for grace and poise—in the 1994 Olympics.

Pork—better known as pig—was his favorite entree—perhaps, because he fancied himself a cannibal, for he looked as what he favored.

2. To separate an appositive (a phrase which renames or describes a noun) that contains commas from the surrounding text:

He was a gourmand—one who readily ate more than what he weighed, given the opportunity—and shamelessly never looked the part.

The car gave an indignant squeal—a sharp, discordant sound that was almost elephantine—and then abruptly cut out.

3. To introduce a restatement, a list, an amplification, or a dramatic shift in tone or thought:

Restatement:

He couldn't have been more than five years old—or at least mentally he couldn't have been, for his mental age preceded his physical age by some centuries.

List:

I had just three items in my suitcase—a flashlight, a shaving kit, and a fresh pair of clothes.

(You could also use a colon in this case, but it would be more formal.)

Amplification:

He was a type of idiot—the very worst!

Dramatic shift:

Oh—hello!

They crept through the creaking hallways of the house, baseball bats and cans of hairspray at the ready—
And there he was!

Sources:

http://academics.smcvt.edu/writingctr/Dashes.htm

http://grammar.quickanddirtytips.com/dashes-grammar.aspx