@Ross at Play
Please clarify this for me.
I thought en-dash and hyphen were the same thing, while em-dash was sometimes just called dash.
Technically, hyphens are identical with a simple dash, while the en-dash and em-dash are only used when publishing documents. The en-dash is defined as being the width of a small letter "n" and the "m" dash is the length of the small letter "m" is which ever font you use, though the majority of fonts don't support either publishing or accent marks. Publishing marks are more precise, so they're more common in printed documents than in electronic documents.
The other uses of publication marks are to denote interrupted speech (only used in fiction and is specifically for dialogue), where the em-dash signifies an incomplete sentence because someone was interrupted. The ellipsis, also separated by spaces on either end--except when there's other punctuation (like a comma) connected to--but just to confuse things--more recent style guides insist you don't use a trailing period (end-stop) following an ellipsis (which is a rule derived from non-fiction uses). Got all that? I know it's confusing.
Thus you have the following:
"That's what I was say--"
"Would you quit yammering? We're sick of listening to you spout off about obscure no one cares about!"
"The rain in Spain--which falls mainly in the plains--is a welcome relief from the oppressive heat."
In this case, the em-dash is largely optional and can generally be replaces with commas (or parenthesis in non-fiction usages). It's mostly used when you want to focus attention on a side-issue which isn't directly related to the topic of the sentence.
"I have something somewhat ... uncomfortable ... to ask."
which is distinct from the ellipses use in non-fiction to denote something removed from the text:
He asked: "I have something ... to ask."
Ellipses (the plural of the singular ellipsis) also denote a "hanging sentence":
"But then, my point is largely ..."
Where, if read aloud, the character's voice would be heard trailing off as they allow the reader to assume what they intended to make (you often hear this after everyone in a conversation wanders away, or a speaker realizes that no one is paying him the slightest attention).
Note: Because I tend to specialize in complex dialogues with multiple characters, I tend to use em-dashes and ellipses a lot, often during arguments where one character will cut another off, or someone will storm out of the room. The punctuation marks are a handy way to denote the action without having to spell out how everyone responds. However, since it's uncommon in normal text, you run the risk that many readers won't understand what they mean, and will naturally assume that an em-dash needs spaces around it, or that an ellipsis means the text is missing a key phrase. :(