Last week's post on colons brought up a question from a commenter about colons and dashes. The difference between the two is subtle: they can both serve to introduce a related element after a sentence, but a dash is a stronger and more informal mark than a colon. Think of a colon as part of the sentence that just ambles along. Susan has two favorite colors (and now I'm going to tell you what they are): orange and blue. A colon informs readers that something more is coming.
A dash, on the other hand—well, it's quite a dramatic thing. A dashing young man is certainly not an ordinary young man, and if you're dashing off to the store, you're not just going to the store, you're going in a hurry. A dash interrupts the flow of the sentence and tells the reader to get ready for an important or dramatic statement. If you added a dash to the "Susan" sentence it would conceptually read something like this: Susan has two favorite colors (wait for it; wait for it!)—orange and blue. Wow!
Given that there isn't anything exciting about Susan's favorite colors, a dash may not be the best choice here, but it wouldn't be wrong. It would be a better choice if that sentence were part of a mystery novel where something orange was missing and Susan was implicated as the thief. Then it could be a dramatic announcement that she loves orange, and a dash would make more sense.
A very important rule about dashes is never, never, never use a hyphen in place of a dash. A hyphen is not a junior dash; it has its own completely separate use.
There is no computer key for a dash; you need to insert a dash as a symbol. If for some reason you can't insert the dash symbol, use two hyphens right next to each other: --.
Dashes can also be used like commas or parentheses to set off part of a sentence. When you use dashes to set off a parenthetical element, you're using the strongest method possible to draw attention to it, so be sure it merits the drama.
Different Types of Dashes
There are two different kinds of dashes: em dashes (—) and en dashes (–). An em dash is longer than an en dash.
(The names come from the fact that historically the em dash was as long as the width of a capital typeset letter M, and the en dash was as long as the width of a capital typeset N. Now with computer typesetting, the widths of each may vary from font to font, with the width on an en dash always falling midway between a hyphen and an em dash.)
The em dash is the kind of dash I referred to in this post; it is the kind of dash you use in a sentence. When people say, "Use a dash," they almost always mean the em dash. The en dash is used much less frequently and usually only to indicate a range of inclusive numbers. You would use an en dash to write something like this:
Susan will be on vacation June 2–June 9.
(Susan will be on vacation June 2 to June 9.)
The to and the en dash between the dates indicate that Susan will not be in the office beginning the second of June and will return on the tenth of June (because an en dash indicates that the numbers are inclusive of those two dates).
Whether using an em dash in a sentence or the shorter en dash to indicate an inclusive range, you can use your own judgment about whether to put spaces between the dash and the words around it—it's a style issue, so just be consistent.
Source: Grammar Girl