Use dashes as you use parentheses: to set off additional information, particularly information that you want to emphasize.

The essay question — worth 50 percent of the whole exam — will be open book.

A dash can also indicate a pause, much as a comma does.

My son wants to buy a car — if he can find an affordable one.

Make a dash by typing two hyphens together. Do not leave any extra spaces around a dash.